Spin Hall Effect (SHE) Assisted Three-Dimensional Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM)

ABSTRACT

The various implementations described herein include methods, devices, and systems for operating magnetic memory devices. In one aspect, a magnetic memory device includes: (1) a core; (2) a plurality of layers that surround the core in succession; (3) a first input terminal coupled to the core and configured to receive a first current, where: (a) the first current flows radially from the core through the plurality of layers; and (b) the radial flow of the first current imparts a torque on, at least, a magnetization of an inner layer of the plurality of layers; and (4) a second input terminal coupled to the core and configured to receive a second current, where: (i) the second current imparts a Spin Hall Effect (SHE) around a perimeter of the core; and (ii) the SHE contributes to the torque imparted on the magnetization of the inner layer by the first current.

RELATED APPLICATIONS

This application is related to U.S. Utility patent application Ser. No.______ (Attorney Docket No. 120331-5002-US) entitled “Three-DimensionalMagnetic Memory Devices,” filed ______,” and U.S. Utility patentapplication Ser. No. ______, (Attorney Docket No. 120331-5003-US)entitled “Methods of Fabricating Three-Dimensional Magnetic MemoryDevices,” filed ______,” each of which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

This relates generally to the field of memory applications, includingbut not limited to magnetic memory.

BACKGROUND

Magnetoresistive random access memory (MRAM) is a non-volatile memorytechnology that stores data through magnetic storage elements. MRAMdevices store information by changing the orientation of themagnetization of a storage layer. For example, based on whether thestorage layer is in a parallel or anti-parallel alignment relative to areference layer, either a “1” or a “0” can be stored in each MRAM cell.

The field of memory applications is becoming more challenging as theperformance requirements for memory-based devices increase. Because ofmany useful properties of MRAM (e.g., retention of data, resistance toerrors, and life span of memory cells), memory systems based on MRAMhave superior performance over conventional memory systems.

SUMMARY

There is a need for systems and/or devices with more efficient,accurate, and effective methods for fabricating and/or operating memorysystems. Such systems, devices, and methods optionally complement orreplace conventional systems, devices, and methods for fabricatingand/or operating memory systems.

The present disclosure describes various implementations of MRAM systemsand devices. As discussed in greater detail below, MRAM stores datathrough magnetic storage elements. These elements typically include twoferromagnetic films or layers that can hold a remnant magnetization andare separated by a non-magnetic material. In general, one of the layershas its magnetization pinned (e.g., a “reference layer”), meaning thatthis layer possesses a large thermal stability and requires a largemagnetic field or spin-polarized current to change the orientation ofits magnetization. The second layer is typically referred to as thestorage, or free, layer and its magnetization direction can be changedby a smaller magnetic field or spin-polarized current relative to thereference layer.

Due to the spin-polarized electron tunneling effect, the electricalresistance of the cell changes due to the relative orientation of themagnetization of the two layers. A memory cell's resistance will bedifferent for the parallel and anti-parallel states and thus the cell'sresistance can be used to distinguish between a “1” and a “0”. Oneimportant feature of MRAM devices is that they are non-volatile memorydevices, since they maintain the information even when the power is off.In particular, the layers can be sub-micron in lateral size and themagnetization direction can still be stable over time and with respectto thermal fluctuations.

In particular, the present disclosure describes a three-dimensional MRAMdevice. In some implementations, the three-dimensional MRAM device is acylindrical Magnetic Tunnel Junction (MTJ) device. Conventional MTJdevices (e.g., MTJs having layers stacked one on top of another) sufferfrom poor thermal stability and data retention as device size decreases.Such a result is problematic because size is a fundamental designconstraint limiting widespread implementation of MRAM (e.g., inhigh-density memory arrays). The three-dimensional geometry (e.g.,cylindrical geometry) of the MTJ described herein allows for asubstantial reduction in device size (e.g., less than 20 nanometers)hardly obtained in conventional MRAM devices. To accomplish thisreduction in device size, the three-dimensional MRAM device includes acylindrical core (e.g., a non-magnetic metal core in the shape of acylinder) and a plurality of layers that surround the core in succession(e.g., two ferromagnetic layers that can hold a magnetic field separatedby a non-magnetic barrier (spacer) material). In such a configuration,each of the plurality of layers is a cylindrical shell with a differentradius. The cylindrical nature of the MTJ facilitates size reductionwhile also maintaining (and in some cases improving) thermal stabilityand data retention of the MTJ.

Additionally, the present disclosure describes a process of fabricatingthe three-dimensional MTJ. The process includes starting with adielectric substrate with a metallic core (e.g., a metal plug)protruding from the dielectric substrate. In such an arrangement, alower portion of the metallic core is not exposed and an upper portionof the metallic core is exposed. The process further includes depositinga first ferromagnetic layer on the exposed portion of the metallic core(and also exposed portions of the dielectric substrate). Next, theprocess includes depositing a non-magnetic spacer layer on exposedsurfaces of the first ferromagnetic layer. Continuing, the processfurther includes depositing a second ferromagnetic layer on exposedsurfaces of the non-magnetic spacer layer. In doing so, thethree-dimensional MTJ includes a cylindrical core and a plurality ofmagnetic and nonmagnetic layers that surround the core in succession.

Additionally, the present disclosure describes a three-dimensional MTJthat uses the Spin Hall Effect (SHE) to reduce an amount of currentneeded to switch a magnetic configuration of the MTJ (switch from aparallel state to an anti-parallel state, or vice versa). Reducing aswitching current (referred to herein as the spin transfer torque (STT)current, the spin-polarized current, and the tunneling current) usingthe SHE, at a minimum, prolongs the life of the MTJ. Additionally, theSpin Hall (SH) spin current can jumpstart the switch from one magneticconfiguration to another (e.g., reduce a switching time as compared toonly using the STT current). To accomplish this, the three-dimensionalSHE MRAM device includes a core and a plurality of layers that surroundthe core in succession, as mentioned above. Additionally, the SHE MRAMdevice includes a first terminal coupled to the core that receives afirst current (e.g., the STT current) and a second terminal couple tothe core that receives a second current (e.g., the SHE current). Thefirst current flows away from the core (e.g., radially) through theplurality of layers and imparts a torque on a magnetization of one ormore of the layers via spin transfer torque. Moreover, the secondcurrent creates a SHE around the perimeter of the core, whichcontributes to the torque imparted by the first current. Due to thecontribution of the SHE, the first current can be reduced. In someimplementations, the first and second terminals are the same terminal.Alternatively, in some implementations, the first and second terminalsare different terminals.

This disclosure addresses the issue of thermal stability loss thatarises with device size reduction in MTJ arrays, which has hampered theimplementation of MRAM as a viable DRAM replacement. For example, inplanar geometries where the magnetization lies in the thin-film plane,thermal stability derives from shape anisotropy, which can be tuned bychanging the in-plane aspect ratio. Scaling this geometry down to sizesless than 30 (or so) nanometers is impractical because large aspectratios are necessary for proper data retention, even at such smallsizes. Similarly, in perpendicular geometries where the magnetizationlies out of the thin-film plane, data retention decreases with the areaof the device and thermal stability arises from interfacial anisotropy.Thus, the thermal stability of both geometries (in-plane andout-of-plane) is severely limited for small device sizes, therebypresenting a challenge for adopting MRAM for applications such as DRAM.

To overcome these issues, a three-dimensional geometry for spin transfertorque (STT) MRAM is described herein that solves the problem of poorthermal stability of the free layer (also referred to as the storagelayer) in magnetic tunnel junction structures with stacked layers. Also,the STT MRAM described herein enables higher data retention inhigh-density memory arrays. In some implementations, the MTJs describedherein have diameters that are less than or approximately equal to 20nanometers.

As will be discusses in further detail below (e.g., with reference toFIG. 5), in some implementations, the storage layer and the referencelayer are concentric cylindrical shells. Moreover, the two layers wraparound a central core in succession, where the central core providesstructural support and serves as current lead if metallic. The storagelayer and the reference layer are separated by a non-magnetic tunnelbarrier (referred to herein as a spacer layer or barrier layer) withhigh spin polarization, such as Magnesium Oxide (MgO). Depending onmagnetic configuration requirements, the reference layer can have asmaller or larger radius relative to a radius of the storage layer. Insome implementations, a current is received by the central core. Thecurrent flows away from the central core through the layers towards theoutermost layer, imparting a torque on the magnetization of the storagelayer and the reference layer via spin transfer torque.

A magnetic ground state (also referred to herein as a magnetizationorientation) of both the storage layer and reference layer can be chosento be either in-plane, out-of-plane (along the axis of the core), orvortex. In the latter case, the magnetization wraps itself around thecore in a clockwise or counterclockwise manner, depending on thecircumstances. In some implementations or instances, both theout-of-plane and vortex magnetic ground states are suitable for writingthe parallel (P) and anti-parallel (AP) configurations provided that thereference layer is in the same magnetic ground state as the storagelayer. In the vortex same magnetic ground state, P and AP correspond tothe two possible chiralities of the storage layer magnetization.

In some implementations, the magnetic ground state of the storage layerand reference layer can be tailored via several parameters, including:(i) exchange energy, (ii) saturation magnetization, (iii) uniaxialanisotropy, (iv) layer thickness, (v) layer height, (vi) radius of thecore, and (vii) core height. Both the exchange energy and the saturationmagnetization depend on material composition. In some implementations,high exchange energy disfavors the vortex magnetic ground state whilehigh magnetization has the opposite effect. Moreover, increasing thevertical height in comparison to the core radius promotes theperpendicular magnetic ground state. Typically, an elongated cylindricalstructure will favor the perpendicular magnetic ground state. Theperpendicular magnetic ground state favors a small curvature radius(e.g., Radius X, FIG. 8A). A flattened cylinder favors either the vortexor in-plane magnetic ground state. In this case, a large curvatureradius (e.g., Radius Y, FIG. 8B) and/or a low exchange constant favorthe vortex magnetic ground state.

In some implementations, the storage and reference layers are made ofthin (0.5-10 nm) CoFeB films with various compositions. In someimplementations, the boron content of the layers varies between 10% and40%. In some implementations, the storage and/or reference layers havethe following composition (Co_(x)Fe_(1-x))_(1-y)B_(y).

If the perpendicular magnetic ground state (along the axis of thecylindrical core) is preferred, the material of the storage and/orreference layers needs to be relatively stiff (e.g., have a largeexchange constant). In some implementations, an increase in cobaltcontent increases the exchange constant. In contrast, if the vortexmagnetic ground state is preferred, layers with a low exchange energy(or high Fe content) are used. In some implementations, exchange energyis decreased by using a combination (bilayer) of CoFeB and other layershaving a lower exchange stiffness, such as permalloy, which lowers theoverall exchange stiffness of the layer.

In some implementations, the storage layer is single layer or acomposite layer using interspersed layers of Tungsten or Tantalum totailor the anisotropy of the storage layer. As explained below, thestorage layer differs from the reference layer because the referencelayer is more thermally stable, which is achieved by changing acomposition and/or the thickness of the reference layer. In someimplementations, the reference layer is made more thermally stable bymaking the reference layer in a synthetic anti-ferromagneticconfiguration where (typically) two ferromagnetic layers are separatedby a thin layer of Ruthenium (or the like). In some implementations, athickness of the Ruthenium layer ranges from 4 to 8 Angstroms. In someimplementations, the layers are coupled via magnetostatic and electroniccoupling (e.g., Ruderman-Kittel-Kasuya-Yosida coupling). The result ofsaid coupling is an increase in the thermal stability of the referencelayer.

Advantages of the three-dimensional MTJ discussed herein include but arenot limited to: (i) higher thermal energy barrier relative to a thermalenergy barrier of a traditional planar geometry MTJ with a similar size(additional increases in the thermal energy barrier can be achieved byincreasing height), (ii) the three-dimensional MRAM device does not relyon interfacial anisotropy for thermal stability as is the case withtraditional perpendicular MTJ's, and therefore the three-dimensional MTJcan use a less complicated/restrictive material set, (iii) thickerferromagnetic layers facilitate increased tunnel magnetoresistanceratios, and (iv) the three-dimensional MRAM device is compatible withultra-dense geometries and lends itself well to three-dimensionalintegration.

This disclosure also addresses issues associated with manufacturing ofthe three-dimensional cylindrical MRAM device. Traditionally,fabrication of the MRAM device begins with a planar complementarymetal-oxide-semiconductor (CMOS) based logic layer, and subsequently thevarious layers are stacked one after another atop the planar CMOS layer.In contrast, fabrication of the three-dimensional cylindrical MRAMdevice begins with a CMOS plug (e.g., the central core) protruding froma dielectric substrate. In some implementations, the CMOS plug is madefrom Tantalum (Ta), Tungsten (W), Copper (Cu), Ruthenium (Ru), andNiobium (Nb), or a combination thereof or a layer of doped Silicon (Si)such as found in vertical transistors. In some implementations, the plugprotruding from the dielectric substrate is fabricated by starting witha dielectric substrate. Next, an opening is formed towards theunderlying CMOS (transistors) layers (e.g., by forming resist patterningvia photolithography and selectively etching the dielectric substrate byusing an anisotropic etching technique, such as reactive-ion etching).In some implementations, the opening is then filled with a metal (e.g.,the plug materials noted above) by electrodeposition and/or a wetsolution based deposition technique. At this stage, the dielectricsubstrate defines a circular hole filled with metal, which is polishedflush with the dielectric substrate. In a subsequent step, thedielectric around the metallic core is removed via selective etchingand/or a dry vacuum-based technique such as reactive-ion etching. Insome implementations, a wet-based technique such as piranha etch is alsoused. Thereafter, the metallic core is left protruding from of thedielectric substrate.

Next, the plurality of layers is deposited on the plug in succession viamagnetron sputtering at normal incidence to the wafer. In someimplementations, the plurality of layers is ordered as follows: astorage layer, a high spin polarization spacer layer (typically MgO),and a reference layer. Alternatively, in some implementations, theplurality of layers is ordered as follows: a reference layer, a highspin polarization spacer layer, and a storage layer. By using magnetronsputtering in combination with the relatively steep plug sidewallangles, it is possible to achieve plug sidewall coverage two to threetimes smaller than the coverage in the field (e.g., exposed surface 1306of the dielectric substrate 1302, FIG. 13) and less than on the top ofthe plug. Consequently, a thickness of the MgO tunnel spacer layerbarrier is two-times thinner on the plug sidewalls as compared to itsthickness on the plug top or field. As a benefit, the tunneling currentvia those thicker regions will be exponentially smaller and thereforewill not contribute to the resistance of the device with the majority ofthe current flowing through the sidewall region.

In a subsequent step, parts of the plurality of layers remaining in thefield can be removed via a self-aligned process that consists ofrepeated ion beam etching (IBE) steps and/or reactive-ion etching (RIE)processes combined with oxide sidewall deposition. For example, an oxidelayer of appropriate thickness is first deposited on the structure, andthen the oxide layer is etched via IBE and/or RIE. This etching removesthe oxide layer on top of the plug as well as the oxide layer in thefield. Furthermore, portions of the plurality of layers on top of theplug and in the field are also removed by the same process but part ofthe oxide layer and the plurality of layers remain unhindered on thesidewalls of the plug. Optionally, the oxide deposition and etch processis repeated to achieve desired results. Thereafter, a physical vapordeposition (PVD) dielectric encapsulation step is performed in such away that the oxide layer in the field is less than the height of theplug. Moreover, in some implementations, the oxide layer on thesidewalls of the plug is removed by etching with IBE at glancingincidence. In some implementations, the oxide layer removal is performedin such a way as to leave some oxide layer on the top of the plug, whichprevents the structure from shorting. In some implementations, a topelectrode is deposited on the top of the plug and patterned (Route 1,FIG. 14C). In some implementations, a variant where the contact runs onthe sidewalls and down to the field is also described herein (Route 2,FIG. 15B).

Advantages of the fabrication process discussed herein include, but arenot limited to: (i) fabrication of cylindrical MRAM devices with highthermal stability at small sizes, (ii) self-aligned process thatrequires one photolithographic step, (iii) process is compatible withhigh density pillar arrays (e.g., an array of cylindrical MTJ can befabricated using this process), (iv) process lends itself well withvertical transistor architectures as the plurality of layers wrap aroundthe vertical transistor channel in some implementations, and (v) nomasking is required.

This disclosure also describes a three-dimensional MRAM device that usesthe Spin Hall Effect (SHE) to reduce a switching voltage. Thethree-dimensional MRAM device has the same structure to thethree-dimensional MRAM device discussed above. However, an additionalcurrent is included (the SHE current), which flows along the centralcore and generates a spin current that imparts a spin torque on thestorage layer. The spin polarization of the SHE-electrons wraps aroundthe central core in a circular manner, akin to one of the possibleground state configurations such as the vortex magnetic ground state ofthe storage layer. In some implementations, the SHE-electrons aretransmitted to the storage layer and impart a torque on the storagelayer. Importantly, the SH current can reduce the STT current withoutthe SHE current passing through the tunnel spacer layer barrier.

Optionally, if the storage layer is in the perpendicular magnetic groundstate, the SHE-electrons provide a spike of orthogonal spin-polarizedelectrons to the storage layer that jumpstart its precession from afirst direction of magnetization to a second direction of magnetization.In some implementations, to maximize the effect of the spike, the SHEcurrent is a short pulse, relative to the precession period of thestorage layer. Optionally, if the storage layer is in the vortexmagnetic ground state, the SHE-electrons impart the same type of spintorque on the storage layer as the STT current, such that the twocontributions—from STT and SHE—can simply be added together. In someimplementations, the effect of the SHE-electrons increases with thelength of the SHE current pulse. It is noted that in the vortex magneticground state, the SHE-electrons will either stabilize (if they have thesame chirality relative to a chirality of the storage layer) or theywill tend to switch the storage layer (if they have the oppositechirality relative to the chirality of the storage layer). In someimplementations, the chirality of the SHE is controlled by controllingthe sign of the current through the core.

It is noted that the circular structure of the three-dimensional MRAMdevice described herein is well suited for the SHE. For example, inplanar geometry MRAM device, SHE-electrons are extracted from one sideof the current-currying lead, and as a result, the SHE-electrons on theother side(s) are effectively wasted. In contrast, the population ofSHE-electrons is transmitted to the storage layer due to the circulargeometry of the three-dimensional MRAM device described herein.Moreover, the three-dimensional MRAM device is a three-terminal device(e.g., a first terminal connected to a first end of the core, a secondterminal connected to a second end of the core, and a third terminalconnected to an outer layer of the MRAM device). The first and secondterminals are used to create the SHE and the third terminal creates theSTT current. Because the STT current passes through the spacer layer, aresistance associated with the STT current is larger than resistancesassociated with the SHE current. Consequently, the STT current passedthrough the magnetic tunnel junction structure is small compared to theSHE currents passed through core. Moreover, from Kirkhoff s law, acurrent originating from the first terminal is approximately the same asa current originating form the second terminal (e.g., treat these twocurrents as the same current: the “Spin Hall current”). Thus, thethree-dimensional MRAM device operates with two different currents: a“Spin Hall” current that flows between first and second terminals, and asmaller current that flows through the magnetic tunnel junctionstructure. In some implementations, the sign of these currentsdetermines their respective direction of flow.

In some implementations, the SHE current pulse coincides with the STTcurrent pulse through the magnetic tunnel junction structure. Moreover,in the case of a perpendicular magnetic ground state, where theSHE-electrons provide orthogonal spins to jumpstart the switchingprocess with a SHE spike, this “spike” occurs at the beginning of theSTT current.

Advantages of the three-dimensional MTJ with the SHE discussed hereininclude but are not limited to: (i) a reduction of the voltagerequirement across the spacer layer, and correspondingly, a reduction ofthe STT current, and (ii) a facilitation of faster switching of thedevice from a first magnetization direction to a second magnetizationdirection (e.g., switching time from state 1012 to state 1014, FIG.10B), especially when the MRAM device is in the perpendicular magneticground state.

In one aspect, some implementations include magnetic memory devicecomprising: (i) a cylindrical core, (ii) a first cylindricalferromagnetic layer that surrounds the cylindrical core, (iii) a spacerlayer that surrounds the first cylindrical ferromagnetic layer; and (iv)a second cylindrical ferromagnetic layer that surrounds the spacerlayer. The cylindrical core, the first cylindrical ferromagnetic layer,the spacer layer, and the second cylindrical ferromagnetic layercollectively form a magnetic tunnel junction.

In another aspect, some implementations include a method of fabricatinga magnetic memory device comprising providing a dielectric substratewith a metallic core protruding from the dielectric substrate, wherein:(i) a first portion of the metallic core is surrounded by the dielectricsubstrate and a second portion of the metallic core protrudes away froma surface of the dielectric substrate, and (ii) the second portion ofthe metallic core comprises: (a) a surface offset from the surface ofthe dielectric substrate and (b) sidewalls extending away from thesurface of the dielectric substrate to the offset surface. The methodfurther includes depositing a first ferromagnetic layer on first exposedsurfaces of the metallic core and the dielectric substrate, depositing aspacer layer on second exposed surfaces of the first ferromagneticlayer, and depositing a second ferromagnetic layer on third exposedsurfaces of the spacer layer. The first ferromagnetic layer, the spacerlayer, and the second ferromagnetic layer each substantially conforms toa shape of the first exposed surfaces.

In yet another aspect, some implementations include magnetic memorydevice comprising: (i) a core, (ii) a plurality of layers that surroundthe core in succession, (iii) a first input terminal coupled to thecore, and (iv) a second input terminal coupled to the core. The firstinput terminal is configured to receive a first current, where (a) thefirst current flows radially from the core through the plurality oflayers and (b) the radial flow of the first current imparts a torque on,at least, a magnetization of an inner layer of the plurality of layers.Further, the second input terminal is configured to receive a secondcurrent, where (a) the second current imparts a Spin Hall Effect (SHE)around a perimeter of the core and (b) the SHE imparted around theperimeter of the core contributes to the torque imparted on themagnetization of the inner layer by the first current. In someimplementations, the plurality of layers includes a first ferromagneticlayer, a spacer layer, and a second ferromagnetic layer, and the innerlayer is the first ferromagnetic layer. Further, in someimplementations, the first ferromagnetic layer is a storage layer andthe second ferromagnetic layer is a reference layer (or vice versa).

Thus, devices and systems are provided with methods for fabricating andoperating magnetic memory, thereby increasing the effectiveness,efficiency, and user satisfaction with such systems and devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

For a better understanding of the various described implementations,reference should be made to the Description of Implementations below, inconjunction with the following drawings in which like reference numeralsrefer to corresponding parts throughout the figures.

FIG. 1A illustrates a schematic diagram of a representative magnetictunnel junction (MTJ) structure in accordance with some implementations.

FIG. 1B illustrates representative energy barriers of the reference andstorage layers of the MTJ of FIG. 1A in accordance with someimplementations.

FIGS. 2A-2B illustrate magnetization orientations in a representativeperpendicular magnetic tunnel junction (pMTJ) structure in accordancewith some implementations.

FIGS. 3A-3D illustrate representative processes for switching the pMTJof FIGS. 2A-2B between the parallel and anti-parallel configurations inaccordance with some implementations.

FIG. 4 is a schematic diagram of a representative spin transfer torque(STT) MRAM device in accordance with some implementations.

FIG. 5 shows an exemplary cylindrical three-dimensional MRAM device inaccordance with some implementations.

FIGS. 6A-6D are cross-sectional views of the cylindricalthree-dimensional MRAM device of FIG. 5 having different magnetizationorientations in accordance with some implementations.

FIGS. 7A-7C illustrate various magnetization orientations for acylindrical MTJ structure in accordance with some implementations.

FIGS. 8A-8B are phase diagrams illustrating the relationship betweendimensions of a cylindrical MTJ structure and magnetization orientationsin accordance with some implementations.

FIGS. 9A-9B illustrate various energy barrier diagrams for thecylindrical MTJ structure in accordance with some implementations.

FIGS. 10A-10B illustrate energy barriers of the cylindrical MTJstructure based on magnetization orientations in accordance with someimplementations.

FIG. 11 provides representative energy barrier equations for variousmagnetization orientations in accordance with some implementations.

FIGS. 12A-12B illustrate various magnetization orientations for acomplete cylindrical MTJ structure in accordance with someimplementations.

FIGS. 13, 14A-C, and 15A-B illustrate a process of fabricating thethree-dimensional MRAM device of FIG. 5 in accordance with someimplementations.

FIG. 16 illustrates an exemplary core used in fabricating thethree-dimensional MRAM device of FIG. 5 in accordance with someimplementations.

FIGS. 17A-17C are flow diagrams showing a method of fabricating athree-dimensional MRAM device, in accordance with some implementations.

FIG. 18 illustrates an exemplary cylindrical Spin Hall Effect (SHE)three-dimensional MRAM device in accordance with some implementations.

FIGS. 19A-19C are cross-sectional views of the cylindrical SHEthree-dimensional MRAM device of FIG. 18 having different magnetizationorientations in accordance with some implementations.

FIG. 20 illustrates representations of switching a ferromagnetic layerfrom a first polarization to a second polarization in accordance withsome implementations.

FIG. 21 provides a diagram showing a relationship between SHE currentdensity and SHE pulse duration.

FIG. 22 is a schematic diagram of relative resistances for the SHE-MRAMdevice of FIG. 18 in accordance with some implementations.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION

Reference will now be made in detail to implementations, examples ofwhich are illustrated in the accompanying drawings. In the followingdetailed description, numerous specific details are set forth in orderto provide a thorough understanding of the various describedimplementations. However, it will be apparent to one of ordinary skillin the art that the various described implementations may be practicedwithout these specific details. In other instances, well-known methods,procedures, components, circuits, and networks have not been describedin detail so as not to unnecessarily obscure aspects of theimplementations.

Conventional MRAM devices (e.g., stacked MTJs) generally have poorthermal stability and data retention when device size is decreased.Cylindrical MRAM devices described herein allow for a substantialreduction in size (e.g., less than 20 nanometers) while also maintaining(and in some cases improving) thermal stability and data retention ofthe MRAM device. An exemplary cylindrical MRAM device includes a centralcore and a plurality of layers that surround the core in succession(e.g., two ferromagnetic layers that can hold a magnetic field separatedby a spacer layer). In some implementations, magnetization orientationof the two ferromagnetic layers is based, at least in part, on thecharacteristics of the two ferromagnetic layers. In someimplementations, the characteristics of the two ferromagnetic layersinclude but are not limited to (i) thicknesses of the first and secondcylindrical ferromagnetic layers and (ii) heights of the first andsecond cylindrical ferromagnetic layers, respectively, impact themagnetization orientation of the two ferromagnetic layers. Additionally,in some implementations, the magnetization orientation of the twoferromagnetic layers is further based on characteristics of thecylindrical core. In some implementations, the characteristics of thecylindrical core include but are not limited to: (i) a radius of thecylindrical core and (ii) a height of the cylindrical core.

FIG. 1A is schematic diagram of a magnetic tunnel junction (MTJ)structure 100 (e.g., for use in an MRAM device) in accordance with someimplementations. In accordance with some implementations, the MTJstructure 100 is composed of a first ferromagnetic layer (referencelayer 102), a second ferromagnetic layer (storage layer 106), and anon-magnetic layer (spacer layer 104). The reference layer 102 is alsosometimes referred to as a pinned or fixed layer. The storage layer 106is also sometimes referred to as a free layer. The spacer layer 104 isalso sometimes referred to as a barrier layer (or a non-magnetic spacerlayer). In some implementations, the spacer layer 104 comprises anelectrically-insulating material such as silicon oxide.

In some implementations, the reference layer 102 and the storage layer106 are composed of the same ferromagnetic material. In someimplementations, the reference layer 102 and the storage layer 106 arecomposed of different ferromagnetic materials. In some implementations,the reference layer 102 is composed of a ferromagnetic material that hasa higher coercivity than the storage layer 106. In some implementations,the reference layer 102 and the storage layer 106 are composed ofdifferent ferromagnetic materials with the same or similar thicknesses(e.g., within 10%, 5%, or 1% of one another). In some implementations,the thickness of the reference layer 102 is different from that of thestorage layer 106 (e.g., the reference layer 102 is thicker than thestorage layer 106). In some implementations, the thickness of the spacerlayer 104 is on the order of a few atomic layers. In someimplementations, the thickness of the spacer layer 104 is on the orderof a few nanometers (nm). In some implementations, thicknesses of thereference layer 102, the spacer layer 104, and the storage layer 106 areuniform. In some implementations, thicknesses of the reference layer102, the spacer layer 104, and the storage layer 106 are not uniform(e.g., a first portion of the spacer layer 104 is thinner relative to asecond portion of the spacer layer 104).

In some implementations, the reference layer 102 and/or the storagelayer 106 is composed of two or more ferromagnetic layers separated fromone another with spacer layers. In some implementations, each of theseferromagnetic layers is composed of identical, or varying, thickness(es)and/or material(s). In some implementations, the spacer layers arecomposed of identical, or varying, thickness(es) and/or material(s) withrespect to one another.

Magnetic anisotropy refers to the directional dependence of a material'smagnetic properties. The magnetic moment of magnetically anisotropicmaterials will tend to align with an “easy axis,” which is theenergetically favorable direction of spontaneous magnetization. In someimplementations and instances, the two opposite directions along an easyaxis are equivalent, and the direction of magnetization can be alongeither of them (and in some cases, about them). For example, inaccordance with some implementations, FIG. 1B shows low energy states114 and 116 corresponding to opposite directions along an easy axis(additional examples are shown in FIGS. 10A-10B with reference to acylindrical three-dimensional MTJ structure).

In some implementations, the MTJ structure 100 is an in-plane MTJ. Inthis instance, the magnetic moments of the reference layer 102 and thestorage layer 106, and correspondingly their magnetization direction,are oriented in the plane of the ferromagnetic films of the referencelayer 102 and the storage layer 106.

In some implementations, the MTJ structure 100 is a perpendicular (orout-of-plane) MTJ. In this instance, the magnetic moments of thereference layer 102 and the storage layer 106, and correspondingly theirmagnetization direction, are oriented perpendicular and out-of-plane tothe ferromagnetic films of the reference layer 102 and the storage layer106.

In some implementations, the MTJ structure 100 has preferred directionsof magnetization at arbitrary angles with respect to the magnetic filmsof the reference layer 102 and the storage layer 106.

In accordance with some implementations, an MRAM device provides atleast two states such that they can be assigned to digital signals “0”and “1,” respectively. One storage principle of an MRAM is based on theenergy barrier required to switch the magnetization of a single-domainmagnet (e.g., switch the magnetization of the storage layer 106) fromone direction to the other.

FIG. 1B shows representative energy barriers of the reference layer 102and the storage layer 106 of the MTJ 100 in accordance with someimplementations. In accordance with some implementations, the energybarrier refers the amount of energy the magnetic material must overcomein order to switch from one magnetization direction to its opposite(e.g., from the state 114 to the state 116). In an MRAM device, themagnetization direction of the reference layer 102 is generallyconsidered fixed, while the magnetization direction of the storage layer106 is varied to store the “0” and “1” states. Accordingly, thereference layer 102 is composed of materials such that an energy barrier112 (EB, ref) of the reference layer 102 is larger than the energybarrier 118 (E_(B,stor)) of the storage layer 106. In particular, FIG.1B shows low energy states 114 and 116 for the reference layer 102separated by the energy barrier 112, and shows low energy states 120 and122 for the storage layer 106 separated by the energy barrier 118. Insome implementations, the storage layer 106 is designed with materialsthat have a magnetic anisotropy that is high enough to store themagnetization over certain time duration (for e.g., 1 week, 1 month, 1year, or 10 years).

For an MRAM device with the MTJ structure 100, the resistance states ofthe MRAM devices are different when the magnetization directions of thereference layer 102 and the storage layer 106 are aligned in a parallel(low resistance state) configuration or in an anti-parallel (highresistance state) configuration, as will be discussed with respect toFIGS. 2A and 2B.

FIGS. 2A-2B illustrate magnetization orientations in a perpendicularmagnetic tunnel junction (pMTJ) structure 200 in accordance with someimplementations. In some implementations, the pMTJ structure 200 is thesame as the MTJ structure 100 presented in FIG. 1A, comprising: thereference layer 102, the spacer layer 104, and the storage layer 106. Insome implementations, the pMTJ structure 200 forms part of a MRAMdevice.

For the pMTJ structure 200 illustrated in FIGS. 2A and 2B, the fixedmagnetization direction 202 for the reference layer 102 is chosen to bein an upward direction and is represented by an up arrow. In someimplementations (not shown), the fixed magnetization direction of thereference layer 102 in the pMTJ structure 200 is in a downwarddirection.

FIG. 2A illustrates the magnetization directions of the storage andreference layers in a parallel configuration. In the parallelconfiguration, the magnetization direction 206 of the storage layer 106is the same as the magnetization direction 202 of the reference layer102. In this example, the magnetization direction 202 of the referencelayer 102 and the magnetization direction 206 of the storage layer 106are both in the upward direction. The magnetization direction of thestorage layer 106 relative to the fixed layer 102 changes the electricalresistance of the pMTJ structure 200. In accordance with someimplementations, the electrical resistance of the pMTJ structure 200 islow when the magnetization direction of the storage layer 106 is thesame as the magnetization direction 202 of the reference layer 102.Accordingly, the parallel configuration is also sometimes referred to asa “low (electrical) resistance” state.

FIG. 2B illustrates the magnetization directions of the storage andreference layers in an anti-parallel configuration. In the anti-parallelconfiguration, the magnetization direction 216 of the storage layer 106is opposite to the “fixed” magnetization direction 202 of the referencelayer 102. In accordance with some implementations, the electricalresistance of the pMTJ structure 200 is high when the magnetizationdirection 216 of the storage layer 106 is the opposite of themagnetization direction 202 of the reference layer 102. Accordingly, theanti-parallel configuration is sometimes also referred to as a “high(electrical) resistance” state.

Thus, by changing the magnetization direction of the storage layer 106relative to that of the reference layer 102, the resistance states ofthe pMTJ structure 200 can be varied between low resistance to highresistance, enabling digital signals corresponding to bits of “0” and“1” to be stored and read. Conventionally, the parallel configuration(low resistance state) corresponds to a bit “0,” whereas theanti-parallel configuration (high resistance state) corresponds to a bit“1”.

Although FIGS. 2A-2B show parallel and anti-parallel configurations withthe pMTJ structure 200, in some implementations, an in-plane MTJstructure, or an MTJ structure with an arbitrary preferred angle, isused instead.

FIGS. 3A-3D illustrate representative processes for switching the pMTJ200 between the parallel and anti-parallel configurations in accordancewith some implementations. In accordance with some implementations,spin-transfer torque (STT) is used to modify the magnetizationdirections of an MTJ. STT is an effect in which the magnetizationdirection of a ferromagnetic layer in an MTJ is modified using aspin-polarized current.

In general, electrons possess a spin, a quantized number of angularmomentum intrinsic to the electron. An electrical current is generallyunpolarized, e.g., it consists of 50% spin up and 50% spin downelectrons. When a current is applied though a ferromagnetic layer, theelectrons are polarized with spin orientation corresponding to themagnetization direction of the ferromagnetic layer, thus producing aspin-polarized current (or spin-polarized electrons).

As described earlier, the magnetization direction of the reference layer102 is “fixed” in an MTJ (e.g., the applied currents are insufficient tochange the magnetization state of the reference layer). Therefore,spin-polarized electrons may be used to switch the magnetizationdirection of the storage layer 106 in the MTJ (e.g., switch betweenparallel and anti-parallel configurations).

As will be explained in further detail, when spin-polarized electronstravel to the magnetic region of the storage layer 106 in the MTJ, theelectrons will transfer a portion of their spin-angular momentum to thestorage layer 106, to produce a torque on the magnetization of thestorage layer 106. When sufficient torque is applied, the magnetizationof the storage layer 106 switches, which, in effect, writes either a “1”or a “0” based on whether the storage layer 106 is in the parallel oranti-parallel configuration relative to the reference layer.

FIGS. 3A-3B illustrate the process of switching from the anti-parallelconfiguration to the parallel configuration. In FIG. 3A, the pMTJstructure 200 is in the anti-parallel configuration, e.g., themagnetization direction 302 of the reference layer 102 is opposite tothe magnetization direction 306 of the storage layer 106.

FIG. 3B shows application of a current such that electrons flow throughthe pMTJ 200 in accordance with electron flow 312. The electrons aredirected through the reference layer 102 which has been magnetized withthe magnetization direction 302. As the electrons flow through thereference layer 102, they are polarized (at least in part) by thereference layer 102 and have spin orientation corresponding to themagnetization direction 302 of the reference layer 102. The majority ofthe spin-polarized electrons tunnel through the spacer layer 104 withoutlosing their polarization and subsequently exert torque on theorientation of magnetization of the storage layer 106. When asufficiently large current is applied (e.g., a sufficient number ofpolarized electrons flow into the storage layer 106), the spin torqueflips, or switches, the magnetization direction of the storage layer 106from the magnetization direction 306 in FIG. 3A to the magnetizationdirection 316 in FIG. 3B.

Thus, as shown in FIG. 3B, the magnetization direction 316 of thestorage layer 106 is in the same (upward) direction as the magnetizationdirection 302 of the reference layer 102. Accordingly, the pMTJstructure 200 in FIG. 3B is in the parallel (low resistance state)configuration. In some implementations and instances, electrons thatpossess spins in the minority (opposite) direction are reflected at thebarrier interfaces and exert torque on the magnetization direction 302of the reference layer 102. However, the magnetization direction 302 ofthe reference layer 102 is not switched because the torque isinsufficient to cause switching in the reference layer 102.

FIGS. 3C-3D illustrate the process of switching from the parallelconfiguration to the anti-parallel configuration. In FIG. 3C, the pMTJstructure 200 is in the parallel configuration. To initiate switching tothe anti-parallel configuration, a current is applied such thatelectrons flow in accordance with electron flow 322 in FIG. 3D. Theelectrons flow from the storage layer 106 to the reference layer 102. Asthe electrons flow through the storage layer 106, they are polarized bythe storage layer 106 and have spin orientation corresponding to themagnetization direction 316 of the storage layer 106.

The MTJ structure 200 in FIG. 3C is in the parallel (low resistancestate) configuration and thus it has lower electrical resistance,therefore, in some implementations and instances, the majority of thespin-polarized electrons tunnel through the spacer layer 104. Minorityspin electrons that are polarized with direction opposite to themagnetization direction 316 of the storage layer 106 are reflected atthe barrier interfaces of the spacer layer 104. The reflected spinelectrons then exert torque on the magnetization 316 of the storagelayer 106, eventually leading to a switch of the magnetization direction316 of the storage layer 106 in FIG. 3C to a magnetization direction 326in FIG. 3D. Thus, the pMTJ structure 200 is switched from the parallel(low resistance state) configuration to the anti-parallel (highresistance state) configuration.

Accordingly, STT allows switching of the magnetization direction of thestorage layer 106. MRAM devices employing STT (e.g., STT-MRAM) offeradvantages including lower power consumption, faster switching, andbetter scalability, over conventional MRAM devices that use magneticfield to switch the magnetization directions. STT-MRAM also offersadvantages over flash memory in that it provides memory cells withlonger life spans (e.g., can be read and written to more times comparedto flash memory).

FIG. 4 is a schematic diagram of a spin transfer torque (STT) MRAMdevice 400 in accordance with some implementations. The includes an MTJdevice with the reference layer 102, the spacer layer 104, the storagelayer 106, and an access transistor 414. The MTJ device is coupled to abit line 408 and a source line 410 via transistor 414, which is operatedby a word line 412. The reference layer 102, the spacer layer 104, andthe storage layer 106 compose the MTJ structure 100 and/or the pMTJstructure 200, as described above with reference to FIGS. 1-3. In someimplementations, the STT-MRAM 400 includes additional read/writecircuitry, one or more additional transistors, one or more senseamplifiers, and/or other components (not shown).

The MTJ structure 100 and/or the pMTJ structure 200 is also sometimesreferred to as an MRAM cell. In some implementations, the STT-MRAM 400contains multiple MRAM cells (e.g., hundreds or thousands of MRAM cells)arranged in an array coupled to respective bit lines and source lines.During a read/write operation, a voltage is applied between the bit line408 and the source line 410 (e.g., corresponding to a “0” or “1” value),and the word line 412 enables current to flow between the bit line 408to the source line 410. In a write operation, the current is sufficientto change a magnetization of the storage layer 106 and thus, dependingon the direction of electron flow, bits of “0” and “1” are written intothe MRAM cell (e.g., as illustrated in FIGS. 3A-3D). In a readoperation, the current is insufficient to change the magnetization ofthe storage layer 106. Instead, a resistance across the MRAM cell isdetermined. e.g., with a low resistance corresponding to a logical “0”and a high resistance corresponding to a logical “1.”

FIG. 5 illustrates a three-dimensional STT MRAM device 500 in accordancewith some implementations (also referred to herein as a cylindrical MRAMdevice or a conical MRAM device). The MRAM device 500 is similar to theMTJ structures and devices explained above with reference to FIGS. 1-4,except that the MRAM device 500 includes a plurality of layers (e.g.,the reference layer 102, the spacer layer 104, and the storage layer106, FIG. 1A) wrapped around a central core 507, thereby forming athree-dimensional cylindrical (or conical) MTJ structure 501. In someimplementations, each of the plurality of layers, when wrapped aroundthe central core 507, is a hollow cylinder (e.g., a cylindrical shell).Alternatively, in some implementations, each of the plurality of layersis a conical shell when wrapped around the central core 507.

The MTJ device 500 includes a core 507, a first cylindricalferromagnetic layer 502, a spacer layer 504, and a second cylindricalferromagnetic layer 506. The first cylindrical ferromagnetic layer 502surrounds the core 507, the spacer layer 504 surrounds the firstcylindrical ferromagnetic layer 502, and the second cylindricalferromagnetic layer 506 surrounds the spacer layer 504. Collectively,the core 507 and the three layers 502, 504, and 506 form the MTJstructure 501. In some implementations, a diameter of the MTJ structure501 is approximately 20 nm. Alternatively, in some implementations, thediameter of the MTJ structure 501 is greater than (or less than) 20 nm.

In some implementations, the core 507, the first cylindricalferromagnetic layer 502, the spacer layer 504, and the secondcylindrical ferromagnetic layer 506 are coaxial (e.g., concentric) withone another. Additionally, in some implementations, heights of the core507 and the three layers 502, 504, and 506 substantially match oneanother (e.g., the core 507 and the three layers 502, 504, and 506 arecoplanar with one another at a first end 605 of the MTJ structure 501and also coplanar with one another at a second end 607 of the MTJstructure 501, FIG. 6A).

In some implementations, the first cylindrical ferromagnetic layer 502is an example of the reference layer 102 and the second cylindricalferromagnetic layer 506 is an example of the storage layer 106.Alternatively, in some implementations, the first cylindricalferromagnetic layer 502 is an example of the storage layer 106 and thesecond cylindrical ferromagnetic layer 506 is an example of thereference layer 102. In some implementations, each of the ferromagneticlayers is composed of identical, or varying, thickness(es) and/ormaterial(s). For example, each of the ferromagnetic layers is made ofCoFeB with various compositions and each has a thickness ranging from0.5 to 10 nm. In some implementations, the boron (B) component for thefirst and/or second ferromagnetic layers varies between 10% and 40%. Insome implementations, the composition of the first cylindricalferromagnetic layer 502 differs from the composition of the secondcylindrical ferromagnetic layer 506. For example, when the firstcylindrical ferromagnetic layer 502 is the storage layer 106, the firstcylindrical ferromagnetic layer 502 may include at least one material(e.g., Tantalum and/or Tungsten) not included in the second cylindricalferromagnetic layer 506. Furthermore, in some implementations, thereference layer 102 (which could be the first cylindrical ferromagneticlayer 502 or the second cylindrical ferromagnetic layer 506, dependingon the circumstances) includes multiple sublayers making the referencelayer 102 more thermally stable relative to a thermal stability of thestorage layer 106. To achieve the increased thermal stability, in someimplementations, the multiple sublayers include two ferromagnetic layersseparated by a layer of Ruthenium (or the like). In someimplementations, a thickness of the Ruthenium layer ranges from 4 to 8angstroms. In some implementations, the multiple sublayers of thereference layer are coupled together using Ruderman-Kittel-Kasuya-Yosidacoupling. It should be noted the ferromagnetic layers may have otherthickness(es) and/or material(s), and the examples provided above areused to provide context.

The spacer layer 504 is an example of the spacer layer 104 (FIG. 1A).The spacer layer 504 is typically made from Magnesium Oxide (MgO) (orthe like). However, in some implementations, the spacer layer 504 ismade from Mg₁-xAl₂-xO₄. In some implementations, the materials used inthe MRAM device 500 are stable at processing temperatures up to 400-425Celsius and stable at operating temperatures up to 125 Celsius.

The reference layer 102, the spacer layer 104, and the storage layer 106are discussed in greater detail above with reference to FIGS. 1A-3D.

The core 507 is disposed along a vertical axis and is used to providestructural support for the MTJ device 500. In some implementations, thecore 507 is made from a metal (e.g., a non-magnetic metal) and serves asa current lead for the MRAM device 500. In some implementations, thecore 507 is made from, at least partially, one or more of Tantalum (Ta),Tungsten (W), Copper (Cu), Ruthenium (Ru), and Niobium (Nb), or acombination thereof In some implementations, the core 507 is conical (orelliptical) in shape (in those implementations, the core 507 is referredto as a conical core 507). Alternatively, in some implementations, thecore 507 is cylindrical in shape (in those implementations, the core 507is referred to as a cylindrical core 507). It is noted that a shape ofthe first cylindrical ferromagnetic layer 502, the spacer layer 504, andthe second cylindrical ferromagnetic layer 506 conforms to an outersurface of the core 507. Thus, when the core 507 is conical in shape,the first cylindrical ferromagnetic layer 502, the spacer layer 504, andthe second cylindrical ferromagnetic layer 506 are also conical inshape.

As explained in more detail below, in some implementations, the core 507receives a current from a source (e.g., via a source line 510), andsubsequently, the current (e.g., electron flow 615, FIG. 6A) flowsradially from the core 507 through the first cylindrical ferromagneticlayer 502 and the spacer layer 504 towards the second cylindricalferromagnetic layer 506. In doing so, the radial flow of the currentimparts a torque on a magnetization of the first cylindricalferromagnetic layer 502 and the second cylindrical ferromagnetic layer506 via spin transfer torque. In those implementations where the firstcylindrical ferromagnetic layer 502 is the storage layer 106, the radialflow of the current is able to flip a polarization of the storage layer106 if the current reaches a threshold current (e.g., the energy barrier118, FIG. 1B; the energy barriers 1006 and 1016, FIGS. 10A-10B). Inthose implementations where the second cylindrical ferromagnetic layer502 is the storage layer 106, the radial flow of the current is able toflip a polarization of the storage layer 106 if the current reaches thethreshold current.

In some implementations, the second ferromagnetic layer 506 receives acurrent from a source (e.g., via a bit line 508), and subsequently thecurrent (e.g., electron flow 617, FIG. 6A) flows from the secondcylindrical ferromagnetic layer 502 through the spacer layer 504 towardsthe first cylindrical ferromagnetic layer 502 and the core 507. In doingso, the flow of the current imparts a torque on a magnetization of thefirst cylindrical ferromagnetic layer 502 and the second cylindricalferromagnetic layer 506 via spin transfer torque.

The MRAM device 500 is also coupled to a bit line 508 and a source line510 via transistor 514, which is operated by a word line 512. In someimplementations, the source line 510 is connected to the core 507 andthe bit line 508 is connected to the second cylindrical ferromagneticlayer 506. Alternatively, in some implementations, the source line 510is connected to the second cylindrical ferromagnetic layer 506 and thebit line 508 is connected to the core 507 (not shown). In someimplementations, the source line 510 is coupled to a top surface of thecore 507. Alternatively, in some implementations (not shown), the sourceline 510 is coupled to a bottom surface of the core 507. Thesecomponents are discussed in further detail above with reference to FIG.4.

For ease of discussion with regards to FIGS. 6A-6D, the firstcylindrical ferromagnetic layer 502 is the storage layer 502 and thesecond cylindrical ferromagnetic layer 506 is the reference layer 506.As such, a radius of the storage layer 502 is less than a radius of thereference layer 506. However, as noted above, a configuration of thestorage layer 502 and the reference layer 506 may be reversed dependingon the circumstances (e.g., a radius of the storage layer 502 is greaterthan a radius of the reference layer 506).

FIGS. 6A-6D illustrate cross-sectional views (taken along line A, FIG.5) of magnetization orientations of the cylindrical MTJ structure 501 inaccordance with some implementations. For ease of illustration anddiscussion, a width of each layer 502, 504, and 506 shown in FIGS. 6A-6Dis the same. However, in some implementations, the width of one or morelayers may differ, depending on the circumstances. For example, thewidth of the reference layer 506 is greater than the width of thestorage layer 502 to increase the thermal stability (e.g., the energybarrier) of the reference layer 506.

FIGS. 6A-6B show cross-sectional views of the cylindrical MTJ structure501 having a perpendicular magnetization orientation (also referred toherein as a perpendicular magnetic ground state). When the cylindricalMTJ structure 501 has the perpendicular magnetization orientation, thecylindrical MTJ structure 501 is classified as a perpendicular MTJ(e.g., similar to the perpendicular MTJ 200, FIGS. 2A-2B). FIGS. 6A-6Billustrate the process of switching from a parallel configuration (FIG.6A) to an anti-parallel configuration (FIG. 6B) when the cylindrical MTJstructure 501 is a perpendicular MTJ. In cross-sectional views 600 and610, the fixed magnetization direction 602 for the reference layer 506is chosen to be in an upward direction and is represented by an uparrow. In some implementations (not shown), the fixed magnetizationdirection of the reference layer 506 is in a downward direction (e.g., adown arrow).

FIG. 6A illustrates the magnetization directions of the storage andreference layers in a parallel configuration. In the parallelconfiguration, the magnetization direction 604 of the storage layer 502is the same as the magnetization direction 602 of the reference layer506. In this example, the magnetization direction 602 of the referencelayer 506 and the magnetization direction 604 of the storage layer 502are both in the upward direction. The magnetization direction of thestorage layer 502 relative to the fixed layer 506 changes the electricalresistance of the cylindrical MTJ structure 501. As discussed above withreference to FIG. 2A, the parallel configuration is also sometimesreferred to as a “low (electrical) resistance” state.

FIG. 6B illustrates the magnetization directions of the storage andreference layers in an anti-parallel configuration. In the anti-parallelconfiguration, the magnetization direction 606 of the storage layer 502is opposite to the “fixed” magnetization direction 602 of the referencelayer 506. As discussed above with reference to FIG. 2B, theanti-parallel configuration is sometimes also referred to as a “high(electrical) resistance” state.

Thus, by changing the magnetization direction of the storage layer 502relative to that of the reference layer 506, the resistance states ofthe cylindrical MTJ structure 501 can be varied between low resistanceto high resistance, enabling digital signals corresponding to bits of“0” and “1” to be stored and read. Conventionally, the parallelconfiguration (low resistance state) corresponds to a bit “0,” whereasthe anti-parallel configuration (high resistance state) corresponds to abit “1”, as discussed above.

Changing the magnetization direction of the storage layer 502 relativeto that of the reference layer 506 is described below with reference toFIGS. 6C-6D.

FIGS. 6C-6D show cross-sectional views of the cylindrical MTJ structure501 having a vortex magnetization orientation (also referred to hereinas a vortex magnetic ground state). When the cylindrical MTJ structure501 has the vortex magnetization orientation, the cylindrical MTJstructure 501 is classified as a vortex MTJ. FIGS. 6C-6D illustrate theprocess of switching from a parallel configuration (FIG. 6C) to ananti-parallel configuration (FIG. 6D) when the cylindrical MTJ structure501 is a vortex MTJ. With vortex MTJs, the magnetization of thecylindrical MTJ structure 501 wraps around the core 507 clockwise (e.g.,a first chirality) or counterclockwise (e.g., a second chirality). Incross-sectional views 620 and 630, the fixed magnetization direction 612for the reference layer 506 is chosen to be going into the page and isrepresented by a solid black dot (e.g., in a counterclockwise manner).In some implementations (not shown), the fixed magnetization directionof the reference layer 506 is coming out of the page and is representedby an “X” (e.g., in a clockwise manner).

FIG. 6C illustrates the magnetization directions of the storage andreference layers in a parallel configuration. In the parallelconfiguration, the magnetization direction 614 of the storage layer 502is the same as the magnetization direction 612 of the reference layer506 (e.g., the chirality of the storage layer 502 is the same as thechirality of the reference layer 506). In this example, themagnetization direction 612 of the reference layer 506 and themagnetization direction 614 of the storage layer 502 are both going intothe page.

FIG. 6D illustrates the magnetization directions of the storage andreference layers in an anti-parallel configuration. In the anti-parallelconfiguration, the magnetization direction 616 of the storage layer 502is opposite to the “fixed” magnetization direction 612 of the referencelayer 506. For example, a chirality of the storage layer 502 differsfrom a chirality of the reference layer 506. Thus, by changing themagnetization direction of the storage layer 502 relative to that of thereference layer 506, the resistance states of the cylindrical MTJstructure 501 can be varied between low resistance to high resistance,enabling digital signals corresponding to bits of “0” and “1” to bestored and read.

As described above with reference to FIGS. 3B-3D, in order to change thecylindrical MTJ structure 501 from a parallel configuration to ananti-parallel configuration (or vice versa), a current (e.g., electronflow 312, FIG. 3B) is applied to the cylindrical MTJ structure 501. Insome implementations, the current is applied through the core 507 (e.g.,via the source line 510, FIG. 5). In those implementations, the receivedcurrent (e.g., electron flow 615) flows radially from the core 507through the storage and spacer layers toward the reference layer 506,and the current 615 imparts a torque on a magnetization of storage layer502 (and also the reference layer 506). When a sufficiently largecurrent is applied (e.g., a sufficient number of polarized electronsflow into the storage layer 502), the spin torque flips, or switches,the magnetization direction of the storage layer 502 from themagnetization direction 614 in FIG. 6C to the magnetization direction616 in FIG. 6D. For example, the current 615 applied to the storagelayer 502 from the core 507 switches a chirality of the storage layer's502 magnetization from a counterclockwise chirality to a clockwisechirality. As a result of said switching, the MTJ structure 501transitions from a parallel configuration to an anti-parallelconfiguration.

In some implementations, the current is applied through the referencelayer 506, as described above with reference to FIGS. 3B and 5. When thecurrent is applied through the reference layer 506 of the cylindricalMTJ structure 501, the current flows from the reference layer 506through the MTJ structure 501 towards the core 507 (e.g., electron flow617, FIG. 6A). When a sufficiently large current is applied (e.g., asufficient number of polarized electrons flow into the storage layer502), the spin torque flips, or switches, the magnetization direction ofthe storage layer 502 from the magnetization direction 614 in FIG. 6C tothe magnetization direction 616 in FIG. 6D.

In some implementations, the current is applied through the referencelayer 506 when the MTJ structure 501 is in the anti-parallelconfiguration, and the current is applied through the core 507 when theMTJ structure 501 is in the parallel configuration (or vice versa). Inaccordance with some implementations, switching configurations isperformed by reversing the flow of the current. Switching from theparallel configuration to the anti-parallel configuration utilizescurrent in one polarity (direction) and switching from the anti-parallelconfiguration back to the parallel configuration utilizes current in theopposite polarity (e.g., current in the opposite direction). To put itin another way: to switch from the parallel configuration to theanti-parallel configuration, the electrons have to flow from the storage(e.g., free) layer to the reference layer, since it is the reflectedelectrons from the minority spin band that cause the storage layer toswitch from the parallel configuration to the anti-parallelconfiguration. Accordingly, switching from the parallel configuration tothe anti-parallel configuration requires the current to flow from thereference layer to the storage layer. To switch from the anti-parallelconfiguration to the parallel configuration, the electrons have to flowfrom the reference layer to the storage layer since it is thetransmitted majority spin up band electrons from the reference layerthat are going to thermalize in the storage layer and impart theirangular momentum. In some implementations, the switch from theanti-parallel configuration to the parallel configuration requires thecurrent to flow from the storage layer to the reference layer.

The discussion above applies equally to FIGS. 6A-6B, and for the sake ofbrevity it is not repeated here. Transitioning from parallel toanti-parallel resistance states (or vice versa) is discussed in greaterdetail above with reference to FIGS. 3A-3D.

FIGS. 7A-7C illustrate various magnetic ground states for MRAM device500 in accordance with some implementations. A magnetic ground statecorresponds to the magnetic anisotropy of a ferromagnetic layer of theMRAM device 500. As explained above with reference to FIG. 1A, amagnetic moment of magnetically anisotropic materials will tend to alignwith an “easy axis,” which is the energetically favorable direction ofspontaneous magnetization. In some implementations and instances, thetwo opposite directions along (or about) an easy axis are equivalent,and the direction of magnetization can be along (or about) either ofthem. As will be described in more detail with reference to FIGS. 8A-8B,the magnetic ground state for a ferromagnetic layer is dictated bycharacteristics of the ferromagnetic layer (e.g., height, thickness, andmaterial composition of the ferromagnetic layer) and the characteristicsof the core 507 (e.g., height, radius, and material composition of thecore 507). For ease of illustration and discussion, the spacer layer 504and the second cylindrical ferromagnetic layer 506 are not shown inFIGS. 7A-7C. However, one skilled in the art will appreciate that thediscussion below applies equally to the second cylindrical ferromagneticlayer 506.

FIG. 7A illustrates a vortex magnetic ground state 700 in accordancewith some implementations. In the vortex magnetic ground state 700(e.g., the vortex magnetization orientation), a magnetic moment 702(e.g., direction of magnetization) of the first cylindricalferromagnetic layer 502 rotates around the core 507. For example, thecore 507 is positioned along an axis 704 and the magnetic moment 702 offirst cylindrical layer 502 rotates around (e.g., about) the axis 704within (e.g., in-plane) the first cylindrical layer 502. In someimplementations, the magnetic moment 702 rotates around the core 507 ina clockwise direction. Alternatively, in some implementations, themagnetic moment 702 rotates around the core 507 in a counterclockwisedirection. Although not shown in FIG. 7A, the magnetic moment 702 of thefirst cylindrical layer 502 rotates around the core 507 through a crosssection of the first cylindrical ferromagnetic layer 502 (e.g., as shownin FIGS. 6C-6D). FIG. 12A illustrates the magnetic moments 1202, 1204 ofthe first cylindrical ferromagnetic layer 502 and the second cylindricalferromagnetic layer 506, respectively, in an anti-parallel configurationwhen the first and second ferromagnetic layers have the vortex magneticground state 700.

FIG. 7B illustrates a perpendicular magnetic ground state in accordancewith some implementations. The arrows 712 represent a direction of themagnetic moment of the bulk material of the first cylindrical layer 512.In some implementations, the magnetic field lines (not shown) extend outof a planar surface 714 of the first cylindrical ferromagnetic layer 502in the same direction represented by the arrows 712 (e.g., upwards) andin doing so, the magnetic moment 712 of the bulk material of the firstcylindrical ferromagnetic layer 502 parallels the axis 704 of the core507. In some implementations, the magnetic moment 712 of the bulkmaterial of the first cylindrical ferromagnetic layer 502 parallels theaxis 704 of the core 507 and the magnetic field in a first direction(e.g., upwards). Alternatively, in some implementations (not shown), themagnetic moment 712 of the bulk material of the first cylindricalferromagnetic layer 502 parallels the axis 704 of the core 507 and themagnetic field in a second direction (e.g., downwards). Although notshown in FIG. 7B, the magnetic moment 712 of the bulk material of thefirst cylindrical ferromagnetic layer 502 extends through a crosssection of the first cylindrical ferromagnetic layer 502 (e.g., as shownin FIGS. 6A-6B). FIG. 12B illustrates the magnetic moments 1212, 1214 ofthe bulk material of the first cylindrical ferromagnetic layer 502 andthe bulk material of the second cylindrical ferromagnetic layer 506,respectively, in an anti-parallel configuration when the first andsecond ferromagnetic layers have the perpendicular magnetic ground state710.

FIG. 7C illustrates an in-plane magnetic ground state in accordance withsome implementations. In the in-plane magnetic ground state, a magneticmoment 722 of the first cylindrical ferromagnetic layer 502 parallelsthe planar surface 714 of the first cylindrical ferromagnetic layer 502.In doing so, the magnetic moment 722 of the first cylindrical layer 502is perpendicular to the axis 704 of the core 507. In someimplementations, the magnetic moment 722 parallels the planar surface714 of the first cylindrical ferromagnetic layer 502 in a firstdirection (e.g., rightwards). Alternatively, in some implementations,the magnetic moment 722 parallels the planar surface 714 of the firstcylindrical ferromagnetic layer 502 in a second direction (e.g.,leftwards). Although not shown in FIG. 7C, the magnetic moment 722 ofthe first cylindrical ferromagnetic layer 502 extends through the crosssection of the first cylindrical ferromagnetic layer 502. Because thestructure possesses radial symmetry, every magnetization direction isenergetically equivalent in the radial plane in the in-plane groundstate. Accordingly, the magnetization direction may be equally likely tobe pointing rightwards or leftwards or in any other direction in theradial plane. In some implementations and situations, this ground state(e.g., having a reference layer with the in-plane magnetic ground state)is not preferred when it comes to encoding information as there is noenergy barrier to overcome to go from the anti-parallel to the parallelconfiguration and the system could assume any angular configurationin-between which is not ideal for storing a bit.

In some implementations, material composition of a ferromagnetic layeris tailored to a specific magnetic ground state. For example,ferromagnetic layers with a lower exchange energy prefer the vortexmagnetic ground state 700 (e.g., lower relative to a baseline). In someimplementations, lowering the exchange energy of a ferromagnetic layeris achieved by increasing and/or decreasing a proportion of one or moreelements/compounds that compose the ferromagnetic layer. For example,increasing a proportion of Fe (e.g., from a baseline) in theferromagnetic layer deceases the exchange energy of the ferromagneticlayer. Alternatively or in addition, lowering the exchange energy of aferromagnetic layer is achieved by using a combination (bilayer) ofCoFeB and other layers, such as permalloy, which lowers the overallexchange stiffness of the layer.

Conversely, in some implementations, ferromagnetic layers with a highexchange energy prefer for the perpendicular magnetic ground state 710.For example, increasing a proportion of Co (e.g., from a baseline) inthe ferromagnetic layer increases an exchange energy of theferromagnetic layer. Other material properties, such as saturationmagnetization and uniaxial anisotropy, are also considered fortailoring.

FIGS. 8A-8B are phase diagrams showing the relationship betweendimensions of the MRAM device 500 and magnetic ground states inaccordance with some implementations. A magnetic ground state of aferromagnetic layer is based, at least in part, on a set ofcharacteristics of the ferromagnetic layer. In some implementations, theset of characteristics includes one or more of: (i) a thickness of theferromagnetic layer, (ii) a height of the ferromagnetic layer, (iii)exchange energy of the ferromagnetic layer, (iv) saturationmagnetization of the ferromagnetic layer, and (v) uniaxial anisotropy ofthe ferromagnetic layer. Additionally, in some implementations, themagnetic ground state of the ferromagnetic layer is further based on aset of characteristics of the core 507. In some implementations, the setof characteristics of the core 507 includes one or more of: (i) a radiusof the core 507 relative to the thickness of the ferromagnetic layer and(ii) a height of the core 507.

A legend 820 illustrates dimensions discussed below with reference tothe phase diagrams 800 and 810. For example, “Radius” is a radius of thecore 507 combined with a thickness of the first cylindrical layer 502.The “Radius” is a fixed dimension (e.g., 5 nm, 7 nm, 10 nm, 15 nm, 20nm, etc.), and therefore an increase in the thickness of the firstcylindrical layer 502 results in a proportional decrease in the radiusof the core 507 (and vice versa). The Y-axis corresponds to a height ofthe cylindrical MTJ structure (e.g., height of the core 507 and firstcylindrical layer 502, also referred to as pillar height) and the X-axiscorresponds to a thickness of the first cylindrical layer 502. In someimplementations, the Y-axis ranges from 0 to 60 nm and the X-axis rangesfrom 0 to 5 nm (of course, these ranges could be increased ordecreased). For ease of illustration and discussion, the spacer layer504 and the second cylindrical layer 506 are not included in FIGS.8A-8B. However, one skilled in the art will appreciate that thediscussion below applies equally to the second cylindrical layer 506.

FIG. 8A is a phase diagram 800 showing the relationship betweendimensions of cylindrical MTJ structures and two magnetic ground states:(i) the perpendicular magnetic ground state 802 and (ii) the parallelmagnetic ground state 804. In this example, the illustrated cylindricalMTJ structures have a Radius of X (which is less than the Radius Y shownin phase diagram 810). The perpendicular magnetic ground state 802 tendsto form in tall (e.g., elongated) cylindrical MTJ structures 501 withthin ferromagnetic layers (e.g., thin relative to a radius of the core507 and/or the Radius of the MTJ structure). In some implementations orinstances, the perpendicular magnetic ground state 802 tends to formwhen a ratio between the pillar height and the thickness of the firstferromagnetic layer 502 satisfies a threshold, where the ratiocorresponds to an energetically favorable direction of spontaneousmagnetization. For example, when the ratio between the pillar height andthe thickness satisfies the threshold, meaning that the firstferromagnetic layer 502 is sufficiently tall and thin, the energeticallyfavorable direction of spontaneous magnetization is along the height(e.g., in a height dimension, as shown by the upward arrows) of thefirst ferromagnetic layer 502. Such is the result because it isenergetically more favorable for the magnetic moment of the firstferromagnetic layer 502 to lie along the axis of the core (in the heightdirection) than it is for the magnetic moment to lie in the plane (e.g.,along the width), based on the dimensions of the first ferromagneticlayer 502 (e.g., the height dimension is the “easy axis”).

In some implementations or instances, the parallel magnetic ground state804 tends to form when the ratio between the pillar height and thethickness does not satisfy the threshold. The in-plane magnetic groundstate 804 favors short cylindrical MTJ structures 501 with thickferromagnetic layers (e.g., thick relative to a radius of the core 507and/or the radius of the MTJ structure). In such cases, it is easier forthe magnetic moment of the first ferromagnetic layer 502 to lieperpendicular to the axis of the core (in the thickness dimension) thanit is for the magnetic moment to lie perpendicular to the axis of thecore, based on the dimensions of the first ferromagnetic layer 502(e.g., the thickness dimension is the “easy axis”).

As shown, the perpendicular magnetic ground state 802 occupies amajority of the phase diagram 800.

FIG. 8B is a phase diagram 810 showing the relationship betweendimensions of cylindrical MTJ structures and two magnetic ground states:(i) the perpendicular magnetic ground state 802 and (ii) the vortexmagnetic ground state 812. In this example, the illustrated cylindricalMTJ structures have a Radius of Y (which is greater than the Radius Xshown in phase diagram 800). The vortex magnetic ground state 812 tendsto form in wider cylindrical MTJ structures 501 (e.g., Radius Y isgreater than 10 nm) with thick ferromagnetic layers (e.g., thickrelative to a radius of the core 507 and/or the Radius of the MTJstructure). Accordingly, when a ratio between the pillar height and thethickness of the first ferromagnetic layer 502 does not satisfy athreshold, meaning that the first ferromagnetic layer 502 issufficiently short and thick, the energetically favorable direction ofspontaneous magnetization is an in-plane rotation around the core 507(e.g., the magnetic moment 702, FIG. 7A).

FIGS. 9A-9B shows representative energy barriers that at least partiallycorrespond to the phase diagrams of FIGS. 8A-8B in accordance with someimplementations. It should be noted that the dimensions for “Height” and“Thickness” shown in FIGS. 9A-9B are merely one set of possibledimensions.

FIG. 9A shows a representative energy barrier 900 that at leastpartially corresponds to the phase diagram 800 of FIG. 8A. An “energybarrier” refers to the amount of energy the magnetic material mustovercome in order to switch from one magnetization direction to itsopposite (e.g., from the state 114 to the state 116, FIG. 1B). Thus, asthe energy barrier for a ferromagnetic layer increases, theferromagnetic layer is said to become more thermally stable. Increasingthe thermal stability of a ferromagnetic layer results in a greaterenergy input being required to switch the magnetization direction of theferromagnetic layer. With reference in FIG. 9A, as pillar height andthickness of the ferromagnetic layer increases, the representativeenergy barrier 900 for the ferromagnetic layer also increases. In thisparticular example, the increase in thermal stability is fairly uniform.

In some implementations, the magnetic ground state of the ferromagneticlayer affects the thermal stability of the ferromagnetic layer. Forexample, if the ferromagnetic layer is in a first magnetic ground state(e.g., the vortex magnetic ground state), then the thermal stability ofthe ferromagnetic layer may differ from a thermal stability of aferromagnetic layer in a second magnetic ground state (e.g., theperpendicular magnetic ground state). To illustrate, with reference toFIG. 9B, the region 904 (dotted circle) shows an energy barrier bulge inthe representative energy barrier 910, which is not present in therepresentative energy barrier 900 (e.g., the region 902 (dotted circle)does not include a corresponding energy barrier bulge and insteadcontinues uniformly upwards toward a peak energy barrier). The energybarrier bulge 904 shown in FIG. 9B, in some circumstances, is caused bythe ferromagnetic layer being in the vortex magnetic ground state 812(FIG. 8B). The energy barrier bulge 904 corresponds to the region 812shown in FIG. 8B.

FIGS. 10A-10B show representative energy barriers of a ferromagneticlayer in different magnetic ground states in accordance with someimplementations. As discussed above, the energy barrier refers theamount of energy the magnetic material must overcome in order to switchfrom one magnetization direction to its opposite (e.g., from the state1002 to the state 1004). FIG. 10A shows low energy states 1002 and 1004for a ferromagnetic layer in a vortex magnetic ground state and multipleenergy barriers 1006-A, 1006-B, and 1006-C. In this example, the lowenergy state is achieved in both 1002 and 1004 magnetic configurations,1002 corresponds to a counterclockwise chirality and 1004 corresponds toa clockwise chirality. 1002 and 1004 have equivalent energies atequilibrium without external perturbations.

FIG. 10B shows low energy states 1012 and 1014 for a ferromagnetic layerin a perpendicular magnetic ground state and multiple energy barriers1016-A and 1016-B. In this example, the low energy state 1012corresponds to a first magnetization direction of the perpendicularmagnetic ground state (e.g., upwards) and the lower energy state 1014corresponds to a second magnetization direction of the perpendicularmagnetic ground state (e.g., downwards). In some implementations, arespective energy barrier for the ferromagnetic layer in the vortexmagnetic ground state differs from a respective energy barrier for theferromagnetic layer in the perpendicular magnetic ground state (e.g.,less energy is required to overcome the energy barrier 1006 relative toan amount of energy required to overcome the energy barrier 1016, orvice versa). It is noted that the illustrated A Energies are notnecessarily drawn to scale.

In some implementations or instances, a first ferromagnetic layer in afirst magnetic ground state with a first set of characteristics has anenergy barrier (e.g., energy barrier 1006-A) that differs from an energybarrier (e.g., energy barrier 1006-B) of a second ferromagnetic layer inthe first magnetic ground state with a second set of characteristics.Put plainly, as discussed above with reference to FIGS. 8A-8B and 9A-9B,an energy barrier for a ferromagnetic layer will differ depending on ageometry of the ferromagnetic layer. To illustrate, FIG. 10A includesthree different energy barriers 1006-A, 1006-B, and 1006-C, whichgradually increase as a result of the geometry of the ferromagneticlayer changing (e.g., increase in layer thickness and/or decrease inlayer height). Further, FIG. 10B includes two different energy barriers1016-A and 1006-B, which gradually increase as a result of the geometryof the ferromagnetic layer changing (e.g., decrease in layer thicknessand/or increase in layer height).

In some implementations, the magnetic ground state of the ferromagneticchanges momentarily from a first magnetic ground state in the low energystates (e.g., vortex magnetic ground state at low energy states 1002 and1004) to a second magnetic ground state in a high energy state (e.g.,perpendicular magnetic ground state at high energy state 1007). Toillustrate this phenomenon, assume the “angle” of the low energy state1002 is “0” degrees and further assume the angle of the low energy state1004 is “180” degrees (e.g., the low energy state 1004 is opposite tothe low energy state 1002). Thus, the midpoint between the two lowenergy states is “90” degrees (e.g., the angle at the high energy stateis perpendicular to the respective angles at low energy states 1002 and1004). Accordingly, as shown in FIG. 10A, the ferromagnetic layermomentarily has the perpendicular magnetic ground state 1007 whenswitching from the counterclockwise chirality to the clockwise chirality(e.g., at the high energy state). A similar result is illustrated inFIG. 10B. For example, the ferromagnetic layer momentarily has eitherthe vortex magnetic ground state 1018 or the parallel magnetic groundstate 1019 when switching from the upwards to downwards.

FIG. 11 provides representative energy barrier equations for variousmagnetization orientations in accordance with some implementations. Theparameters labeled in FIG. 11 (e.g., exchange energy, demagnetization(demag) anisotropy, uniaxial anisotropy, inner diameter, and externaldiameter) relate to characteristics of the first ferromagnetic layer, asdiscussed above (e.g., magnetization orientation tailoring).Additionally, changing one or more of the parameters in the variousequations may result in an energy barrier for the first ferromagneticlayer also changing (e.g., as shown in FIGS. 10A-10B). For ease ofillustration and discussion, the barrier layer 504 and the secondcylindrical layer 506 are not included in FIG. 11. However, one skilledin the art will appreciate that the equations apply equally to thesecond ferromagnetic layer 506.

FIGS. 13-15 illustrate a process of fabricating the three-dimensionalMRAM device in accordance with some implementations. Each of the viewsshown in FIGS. 13-15 is a cross-sectional view of the three-dimensionalMRAM device during the fabrication process.

FIG. 13 illustrates several initial steps in the fabrication process.The process begins (at step 1300) with providing a dielectric substrate1302 with a metallic core 1304 protruding from the dielectric substrate1302. In some implementations, the metallic core 1304 is a CMOS plugmade from Tantalum (Ta), Tungsten (W), Copper (Cu), Ruthenium (Ru), andNiobium (Nb), or a combination thereof. In some implementations,providing the dielectric substrate 1302 with the protruding metalliccore 1304 includes forming the dielectric substrate (e.g., thermallyformed silicon oxide, silicon nitride, silicon carbide, silicon oxide,or a combination thereof, deposited by plasma-enhanced chemical vapordeposition) and defining an opening 1305 in the underlying CMOS layers(e.g., by creating resist patterning via photolithography and/orselectively etching the dielectric substrate 1302 by using ananisotropic etching technique such as reactive ion etching (RIE)). Insome implementations, the opening 1305 defined by the dielectricsubstrate 1302 is filled with a metal (e.g., by electrodeposition and/ora wet solution based deposition technique), thereby forming the metalliccore 1304. In some implementations, a chemical mechanical polishing(CMP) operation removes excess metal deposited on the dielectricsubstrate 1306.

At this stage, the dielectric substrate defines an opening 1305 (e.g.,circular, or some other shape) filled with metal (e.g., the metalliccore 1304), which is polished flush with the dielectric substrate 1302.Next, portions of the dielectric substrate 1302 around the metallic core1304 are removed (e.g., via selective etching and/or dry vacuum-basedtechniques such as RIE). In some implementations, a wet-based techniquesuch as piranha etch is also used. Thereafter, the metallic core 1304 isleft protruding out of the dielectric substrate 1302 (as shown at step1300). In some implementations, providing the metallic core 1304 and thedielectric substrate 1302 further includes providing both in a vacuumchamber (e.g., a vacuum chamber used to during physical vapor depositionand/or sputtering processes). The protruding core 1304 is sometimesreferred to herein as a plug.

FIG. 16 illustrates an exemplary metallic core 1304 without thedielectric substrate 1302. The metallic core 1304 includes an exposedportion 1602 and an unexposed portion 1604. The unexposed portion 1604is surrounded by the dielectric substrate 1302 (e.g., the portion thatremains in the opening 1305) and the exposed portion 1602 is thatprotrudes away from the exposed surface 1306 of the dielectric substrate1302 (the exposed surface 1306 is sometimes referred to as the field).The metallic core 1304 also includes (i) a surface 1606 offset from theexposed surface 1306 of the dielectric substrate 1302 and (ii) asidewall 1608 extending away from the exposed surface 1306 of thedielectric substrate 1302 to the offset surface 1606. In someimplementations (not shown), the sidewall 1608 is perpendicular to theexposed surface 1306 of the dielectric substrate 1302 (e.g., the core1304 is a cylindrical core). Alternatively, in some implementations, thesidewall 1608 is angled/slanted (α) relative to the exposed surface 1306of the dielectric substrate 1302 (e.g., the core 1304 is a conicalcore). In some implementations, the angle (α) ranges from about 5 to 20degrees.

Turning back to FIG. 13, the process further includes depositing (1310-Aor 1310-B) a plurality of layers onto the exposed portion 1602 of themetallic core 1304 and the exposed surface 1306 of the dielectricsurface 1302. In some implementations, the depositing is performed usinga physical vapor deposition (PVD) technique (e.g., via PVD sputteringand/or evaporation). In some implementations, the PVD 1310-A is anangled PVD 1312 (e.g., a glancing incidence) where the core 1304 and thesubstrate 1302 rotate about an axis 1313 during the PVD 1310-A.Alternatively, in some implementations, the PVD 1310-B is a direct PVD1314 (e.g., a normal incidence) where the core 1304 and the substrate1302 rotate about the axis 1313 so as to maintain a high uniformitythroughout the wafer.

In some implementations, depositing the plurality of layers includes:(i) depositing a first ferromagnetic layer 502 on the exposed portion1602 of the metallic core 1304 and the exposed surface 1306 of thedielectric substrate 1302. After depositing the first ferromagneticlayer 502, the first ferromagnetic layer 502 has exposed surfaces.Accordingly, the process further includes depositing a spacer layer 504on the exposed surfaces of the first ferromagnetic layer 502. Afterdepositing the spacer layer 504, the spacer layer 504 has exposedsurfaces. Accordingly, the process further includes depositing a secondferromagnetic layer 506 on the exposed surfaces of the spacer layer 504.In some implementations (not shown), the second ferromagnetic layer 506consists of multiple sublayers. For example, the multiple sublayersinclude a layer of Ruthenium (or another element or compound withsimilar properties) sandwiched by two ferromagnetic layers. Thesublayers of the second ferromagnetic layer 506 are discussed in furtherdetail above with reference to FIG. 5.

The resulting structure after depositing (using either 1310-A or 1310-B)the plurality of layers is shown at step 1320. As shown, three layers502, 504, and 506 have been deposited on the exposed surfaces of themetallic core 1304 and the dielectric substrate 1302 in succession. Insome implementations, a thickness of each layer varies (or in someimplementations the thickness of each layer is the same). For example,the first ferromagnetic layer 502 is thinner that the secondferromagnetic layer 506 (or vice versa). In another example, the spacerlayer 504 in thinner than the two ferromagnetic layers (or vice versa).Additionally, in some implementations, a thickness of each layer variesalong a length of the layer. For example, the spacer layer 504 isthicker on the exposed surface 1306 of the dielectric substrate 1302 andthe offset surface 1606 of the core 1304, relative to a thickness of thespacer layer 504 along the sidewall 1608 of the core 1304 (in someimplementations, the same is true for the two ferromagnetic layers). Inthose implementations where the spacer layer 504 is thinner along thesidewall 1608 of the core 1304, a tunneling current at the thickerregions of the spacer layer 504 is exponentially smaller relative to atunneling current at the thinner regions of the spacer layer 504. As aresult, the tunneling current at the thicker regions does not(substantially) contribute to the resistance of the MRAM device 500 as amajority of the tunneling current flows through the thinner sidewallregion of the spacer layer 504.

FIGS. 14A-14C illustrate a first option for finishing the MRAM device inaccordance with some implementations. The first option corresponds toFIG. 17B.

In the first option, the process includes depositing 1400 an insulatinglayer (e.g., an oxide 1412) on exposed surfaces of the secondferromagnetic layer 506. In some implementations, the depositing 1400 isachieved using PVD 1402 (or the like). After the PVD 1402, the structure1410 is achieved. Thereafter, the process further includes removing 1420portions of the deposited layers at predetermined locations (e.g.,selective removal). For example, the removing 1420 removes portions ofthe first ferromagnetic layer 502, the spacer layer 504, the secondferromagnetic layer 506, and the insulating layer 508 from the offsetsurface 1606 and the field 1306. In some implementations, the removingis achieved using ion beam etching (IBE) and/or RIE processes. In someimplementations, the IBE and/or RIE processes is/are performed at anormal incidence.

In some implementations, the removing 1420 creates and exposes ends 1424of the plurality of layers. For example, the removing 1420 at least: (i)creates and exposes an end of the first ferromagnetic layer 502, and(ii) creates and exposes an end of the second ferromagnetic layer 506.Moreover, the removing 1420 creates a structure that substantiallymirrors a shape of the exposed portion 1602 of the core 1304 (e.g., theresulting structure shown in FIG. 14A is conical in shape). In someimplementations, steps 1400 and 1420 are repeated one or more timesuntil a desired result is achieved.

In some implementations, the process further includes depositing 1430 anadditional insulating layer (e.g., additional dielectric 1412) onsurfaces exposed by the removing 1420 (e.g., using PVD 1432). Forexample, the depositing 1430 includes at a minimum depositing 1432 thedielectric 1412 on the exposed ends of the first and secondferromagnetic layers, respectively, to electrically insulate themetallic core 1304, the first ferromagnetic layer 502, the spacer layer504, and the second ferromagnetic layer 506 from one another. In someimplementations. The dielectric 1412 is an oxide material (e.g., SiO₂,Al₂O₃). In some implementations, the dielectric is a nitride material(e.g., Si₃N₄, SiN_(x), TiN etc.). In some implementations, thedielectric 1412 is any other applicable dielectric (e.g., DLC).

In some implementations, the process further includes removing 1440(e.g., etching, ablating, etc.) portions of the newly depositedinsulating layer 1412 to expose, at least partially, a sidewall 1442 ofthe second ferromagnetic layer 506. In some implementations, theremoving is performed using IBE and/or RIE processes 1444 (shown withstep 1430 for ease of illustration). In some implementations, the core1304 and the substrate 1302 rotate about the axis 1313 during the IBEand/or RIE processes 1444. In some implementations, a direction of theion beam 1444 is substantially perpendicular to the sidewall 1608 of themetallic core 1304 during the rotating (e.g., a glancing incidence). Insome implementations, the etching 1444 is combined with a chemicallysensitive endpoint technique such as a secondary mass ion spectroscopytechnique.

In some implementations, processes 1430 and 1440 are repeated one ormore times until a desired result is achieved. An exemplary desiredresult in shown at step 1450. There, a sidewall 1452 of the secondferromagnetic layer 506 is partially exposed.

The process further includes depositing 1460 (e.g., using PVD or thelike) a metal contact 1462 on the insulator layer 1412, where a shape ofthe metal contact 1462 substantially complements a shape of theinsulator layer 1412 (e.g., complements the shape of the oxide 1412shown at step 1450). Moreover, complementary sidewall portions 1463 ofthe metal contact 1462 contact 1464 the partially exposed sidewall 1452of the second ferromagnetic layer 506. In doing so, an electricalconnection is made between the metal contact 1462 and the secondferromagnetic layer 506. Additionally, due to the remaining portions ofthe oxide 1412, the metal contact 1462 is insulated from othercomponents of the MRAM device (e.g., electrically insulated from thefirst ferromagnetic layer 502, the spacer layer 504, and the metalliccore 1304). Due to the successive arrangement of the layers, themetallic core 1304 only contacts the first ferromagnetic layer 502. Insome implementations (not shown), the metal contact 1462 is connected toa terminal. For example, the metal contact 1462 is connected to the bitline 508. Alternatively, in a different example, the metal contact 1462is connected to the source line 510. Although not shown, the metalliccore 1304 is also connected to a terminal (e.g., the bit line 508 or thesource line 510).

FIGS. 15A-15B illustrate a second option for finishing the MRAM devicein accordance with some implementations. The second option correspondsto FIG. 17C.

In the second option, the process includes depositing 1500 an insulatinglayer (e.g., an oxide 1512) on exposed surfaces of the secondferromagnetic layer 506. In some implementations, the depositing 1500 isachieved using PVD 1502 or the like. Thereafter, the process furtherincludes removing 1510 portions of the deposited layers. For example,the removing 1510 removes portions of the first ferromagnetic layer 502,the spacer layer 504, the second ferromagnetic layer 506, and theinsulating layer 1512 from the offset surface 1606 and the field 1306(result shown at 1520). In some implementations, the removing isperformed using IBE and/or RIE processes, or the like (as discussedabove with reference to FIG. 14A).

In some implementations, the removing 1510 creates and exposes ends 1524of the plurality of layers. For example, the exposed ends include atleast: (i) an end of the first ferromagnetic layer 502, and (ii) an endof the second ferromagnetic layer 506. Additionally, the removing 1510removes portions of the insulating layer 1512 to expose, at leastpartially, a sidewall 1522 of the second ferromagnetic layer 506. Theremoving 1510 creates a structure that substantially mirrors a shape ofthe exposed portion 1602 of the core 1304 (e.g., the resulting structureshown at step 1520 is conical in shape). In some implementations, steps1500 and 1510 are repeated one or more times until a desired result isachieved.

The process further includes depositing 1530 a metal layer 1532 onsurfaces newly exposed by the removing 1510. In some implementations,the newly exposed surfaces include, at a minimum, (i) the offset surface1606 of the core 1304, (ii) the partially exposed sidewall 1522 of thesecond ferromagnetic layer 506, and (iii) the respective ends 1524 ofthe first and second ferromagnetic layers. In some implementations, themetal layer 1532 substantially conforms to a shape of the newly exposedsurfaces and the remaining oxide 1512. In some implementations, thestructure is subsequently encapsulate using another dielectric layer andthen the contact at the top of the pillar removed by polishing usingchemical-mechanical planarization (CMP) and a possible IBE touch up.

The process further includes, removing 1540 portions of the metal layer1532 that contact (i) the offset surface 1606 of the core 1304 and (ii)the respective ends 1524 of the first and second ferromagnetic layers.As shown, the metal layer 1532 remains in contact 1542 with thepartially exposed sidewall 1522 of the second ferromagnetic layer 506.Moreover, due to the remaining portions of the oxide 1512, the metallayer 1532 is insulated from other components of the MRAM device (e.g.,electrically insulated from the first ferromagnetic layer 502, thespacer layer 504, and the metallic core 1304). Due to the successivearrangement of the layers, the metallic core 1304 only contacts thefirst ferromagnetic layer 502.

In some implementations, the metallic core 1304 is connected to a firstterminal (e.g., input terminal 1542), and the second ferromagnetic layer506 is connected to a second terminal (e.g., output terminal 1544) viathe metal layer 1532. Although the core 1304 in FIG. 15B is connected tothe input terminal 1542 and the metal layer 1532 is connected to theoutput terminal 1544, in some implementations, the input terminal 1542is connected to the metal layer 1532 and the core 1304 is connected tothe output terminal 1544. Although not shown about with reference toFIGS. 13-15A, the overall structure shown in FIG. 15B may apply equallyto the MRAM devices illustrated in FIGS. 13-15A. The elongateddielectric substrate 1302 is not shown in FIGS. 13-15A for ease ofillustration. It is noted that the output terminal 1544 in someimplementations is not within the dielectric substrate 1302, but may beconnected to the metal layer 1532 at some other location.

In some implementations, the process illustrated and described abovewith reference to FIGS. 13-15 can be implemented to create an array(e.g., thousands or millions) of MRAM devices. For example, a singledielectric substrate (or multiple substrates positioned adjacent to oneanother) may be provided with multiple metallic cores (e.g., hundreds orthousands plugs) protruding in a grid-like fashion from the dielectricsubstrate. In such an arrangement, the steps described above are appliedto the metallic cores to form the array. For example, the depositingoperation (step 1310-A or 1310-B) would deposit a plurality of layers oneach of the metallic cores and the removing operation (step 1420 or1510) would remove portions of the deposited layers to isolate each corefrom one another. The remaining steps could then be implemented tofinish the array. In some implementations, spacing (e.g., pitch) betweeneach of the plugs ranges from 10 to 100 nm, and the lateralsize/diameter of each finished MRAM devices ranges from about 7 to 20nm.

FIGS. 17A-17C are flow diagrams showing a method 1700 of fabricating athree-dimensional MRAM device, in accordance with some implementations.The three-dimensional MRAM device may be an example of thethree-dimensional MRAM device 500 and/or the three-dimensional MRAMdevice 1800.

The method 1700 includes (1702) providing a dielectric substrate (e.g.,dielectric substrate 1302, FIG. 13) with a metallic core (e.g., core1304, FIG. 13) protruding from the dielectric substrate. A first portion(e.g., unexposed portion 1604, FIG. 16) of the metallic core issurrounded by the dielectric substrate and a second portion (e.g.,exposed portion 1602, FIG. 16) of the metallic core protrudes away froma surface (e.g., exposed surface 1306, FIG. 13) of the dielectricsubstrate (1704). Additionally, the second portion of the metallic corecomprises: (i) a surface (e.g., surface 1606, FIG. 16) offset from thesurface of the dielectric substrate and (ii) sidewalls (e.g., sidewall1608, FIG. 16) extending away from the surface of the dielectricsubstrate to the offset surface (1706). In some implementations, thesecond portion of the metallic core is conical or cylindrical in shape.Providing the dielectric substrate with the metallic core is describedin further detail above with reference to step 1300 (FIG. 13).

In some implementations, the dielectric substrate is positioned along afirst axis, the metallic core is positioned along a second axis, and thefirst axis is substantially orthogonal to the second axis. For example,with reference to FIG. 13, the core 1304 is positioned along the axis1313, and the dielectric substrate 1302 is substantially orthogonal tothe axis 1313, and is therefore positioned along a different axis (notshown).

In some implementations, the surface (e.g., exposed surface 1306, FIG.13) of the dielectric substrate is a first surface, the dielectricsubstrate includes a second surface that is opposite to the firstsurface, and a bottom surface (e.g., a surface opposite the offsetsurface 1606, FIG. 16) of the metallic core and the second surface ofthe dielectric substrate are coplanar.

In some implementations, the sidewalls of the second portion of themetallic core are slanted relative to the surface of the dielectricsubstrate (e.g., slanted at angle (α), FIG. 16). Alternatively, in someimplementations, the sidewalls of the second portion of the metalliccore are perpendicular to the surface of the dielectric substrate.

The method 1700 further includes depositing (1708) a first ferromagneticlayer (e.g., the ferromagnetic layer 502, FIG. 13) on first exposedsurfaces of the metallic core and the dielectric substrate. In someimplementations, the first exposed surfaces comprise: (i) the offsetsurface of the metallic core, (ii) the sidewalls in the second portionof the metallic core, and (iii) the surface of the dielectric substrate.When the first ferromagnetic layer is deposited, a first surface of thelayer contacts the first exposed surfaces of the metallic core and thedielectric substrate. Thus, the first surface of the first ferromagneticlayer is an unexposed surface. Further, when the first ferromagneticlayer is deposited, a second surface of the layer opposite the firstsurface becomes exposed, thereby forming the second exposed surfaces.Each deposited layer has the same exposed/unexposed configuration.

The method 1700 further includes depositing (1710) a spacer layer onsecond exposed surfaces of the first ferromagnetic layer. The spacerlayer may be an example of the spacer layer 504 (FIGS. 5 and 13).

The method 1700 further includes depositing (1712) a secondferromagnetic layer (e.g., the ferromagnetic layer 506, FIG. 13) onthird exposed surfaces of the spacer layer. After depositing the secondferromagnetic layer, the structure shown at step 1320 is achieved. Asshown, the first ferromagnetic layer, the spacer layer, and the secondferromagnetic layer each substantially conforms to a shape of the firstexposed surfaces.

The three depositing steps 1708, 1710, and 1712 are illustrated as asingle operation at either step 1310-A or 1310-B. As described abovewith reference to FIG. 13, the depositing operation at step 1310-Ainvolves depositing the respective layers at a glancing incidence whilerotating the MRAM device about the axis 1313. In contrast, thedepositing operation at step 1310-B involves depositing the respectivelayers at a normal incidence with no rotation. The result of either step1310-A or 1310-B is shown at step 1320.

In some implementations, providing the metallic core and the dielectricsubstrate comprises providing the metallic core and the dielectricsubstrate in a vacuum chamber. Further, each depositing operation isperformed using a physical vapor deposition process within the vacuumchamber.

The method 1700 further includes depositing (1714) an insulating layer(e.g., oxide 1412, FIG. 14A; oxide 1512, FIG. 15A) on fourth exposedsurfaces of the second ferromagnetic layer. The result of step 1714 isshown at step 1410 (FIG. 14A) and step 1510 (FIG. 15). The followingsteps illustrate two different routes to finish fabrication of the MRAMdevice. The first route is provided in FIG. 17B and the second route isprovided in FIG. 17C. FIG. 17B relates to FIGS. 14A-14C, while FIG. 17Crelates to FIGS. 15A and 15B.

Turning to FIG. 17B, in some implementations, the method 1700 furtherincludes removing (1716) portions of the first ferromagnetic layer, thespacer layer, the second ferromagnetic layer, and the insulating layer.In some implementations, the removing, at least: (i) creates and exposesan end of the first ferromagnetic layer, and (ii) creates and exposes anend of the second ferromagnetic layer. In some implementations, theremoving comprises etching the first ferromagnetic layer, the spacerlayer, the second ferromagnetic layer, and the insulating layer using anion-beam etching and/or a chemically-reactive plasma. For example, withreference to FIG. 14A, at step 1420 the IBE 1422 is used to removeselect portions of the deposited layers to expose ends 1424 of thelayers adjacent to the offset surface 1606 of the core 1304.Additionally, the IBE 1422 removes select portions of the depositedlayers on the exposed surface 1306 of the dielectric substrate 1302.This removal is particularly important when fabricating an array of MRAMdevices because the removal isolates each of the MRAM devices in thearray. In this way, each MRAM device in the array can be programmedindividually as either a “0” or a “1.”

In some implementations, steps 1714 and 1716 are repeated one or moretimes until a desired result is achieved.

Continuing, in some implementations, the method 1700 further includesdepositing (1718) a second insulating layer on fifth exposed surfaces,including the exposed ends of the first and second ferromagnetic layers,respectively, to electrically insulate the metallic core, the firstferromagnetic layer, the spacer layer, and the second ferromagneticlayer from one another. In some implementations, a thickness of thesecond insulating layer paralleling the sidewalls of the second portionof the metallic core is less than other thicknesses of the secondinsulating layer. For example, with reference to FIG. 14B, at step 1430a PVD process 1432 deposits the oxide 1412 onto the structure thatresulted from step 1420. The oxide 1412 includes a notch 1413 making athickness of the oxide 1412 paralleling the sidewall of the metalliccore be less than other thicknesses of the oxide 1412.

In some implementations, the method 1700 further includes removing(1720) portions of the second insulating layer to expose, at leastpartially, a sidewall of the second ferromagnetic layer. For example, atstep 1440 a sidewall IBE 1444 (or the like) removes portions of theoxide 1412 to expose the sidewall 1452 of the second ferromagnetic layer506. In some implementations, the method 1700 further includes rotatingthe metallic core while removing (1720) the portions of the secondinsulating layer to partially expose the sidewall of the secondferromagnetic layer (e.g., rotate about the axis 1313, FIG. 14B).Moreover, in some implementations, the removing comprises etching (e.g.,ablating) the second insulating layer using ion-beam etching and/or achemically-reactive plasma, where a direction of the ion beam issubstantially perpendicular to the sidewalls of the second portion ofthe metallic core during the rotating (e.g., a glancing incidence). Insome implementations, steps 1718 and 1720 are repeated one or more timesuntil a desired result is achieved.

In some implementations, the method 1700 further includes, after theremoving (1720), depositing (1722) a metal contact on the secondinsulator layer, as shown at step 1460 (FIG. 14C). As shown in FIG. 14C,a shape of the metal contact 1462 substantially complements a shape ofthe second insulator layer 1412. Moreover, complementary sidewallportions 1463 of the metal contact 1462 contact 1464 the partiallyexposed sidewall 1452 of the second ferromagnetic layer 506. In doingso, an electrical connection is made between the metal contact 1462 andthe second ferromagnetic layer 506. Additionally, due to the remainingportions of the oxide 1412, the metal contact 1462 is insulated fromother components of the MRAM device (e.g., electrically insulated fromthe first ferromagnetic layer 502, the spacer layer 504, and themetallic core 1304).

Turning to FIG. 17C, in some implementations, the method 1700 furtherincludes removing (1724) portions of the first ferromagnetic layer, thespacer layer, the second ferromagnetic layer, and the insulating layer,where the removing at least (i) exposes the offset surface of themetallic core and (ii) partially exposes a sidewall of the secondferromagnetic layer (e.g., exposed sidewall 1522, FIG. 15A). Forexample, the IBE process 1514 (or the like) removes select portions ofthe deposited layers to expose ends 1524 of the layers adjacent to theoffset surface 1606 of the core 1304. Additionally, the IBE 1514 removesselect portions of the deposited layers on the exposed surface 1306 ofthe dielectric substrate 1302.

In some implementations, steps 1714 and 1724 are repeated one or moretimes until a desired result is achieved.

In some implementations, the method 1700 further includes depositing(1726) a metal layer on surfaces newly exposed by the removing. In someimplementations, the newly exposed surfaces includes: (i) the offsetsurface of the cylindrical core, (ii) the partially exposed sidewall ofthe second ferromagnetic layer, and (iii) the respective ends of thefirst and second ferromagnetic layers. For example, the metal layer 1532is deposited on the structure at step 1530. As shown in FIG. 15B, themetal layer 1532 substantially conforms to a shape of the newly exposedsurfaces.

In some implementations, the method 1700 further includes removing(1728) portions of the metal layer 1530 that contact (i) the offsetsurface of the cylindrical core and (ii) the respective ends of thefirst and second ferromagnetic layers. For example, the portions removedduring step 1728 can be determined by comparing the structures shown atsteps 1530 and 1540 (FIG. 15B). Importantly, the metal layer 1532remains in contact 1542 with the partially exposed sidewall 1522 of thesecond ferromagnetic layer 506. Moreover, the oxide 1512 prevents themetal layer 1532 from contacting the spacer layer 504, the firstferromagnetic layer 502, and the core 1304.

Further, as shown in FIG. 15B, the metallic core is connected to a firstterminal (e.g., input terminal 1542) and the second ferromagnetic layeris connected to a second terminal (e.g., output terminal 1544) via themetal layer 1532.

In some implementations, the steps of the method 1700 may be repeatedsuch that additional three-dimensional MRAM devices are fabricated. Inaddition, in some implementations, the method 1700 further includesforming an array of three-dimensional MRAM devices. Moreover, in someimplementations, the dielectric substrate is a dielectric substrateassociated with each three-dimensional MRAM devices in the array ofthree-dimensional MRAM devices. Alternatively, in some implementations,each three-dimensional MRAM device includes a distinct dielectricsubstrate.

The array of three-dimensional MRAM devices may be interconnect viabusing (or other forms of electrical contacts and terminals) and mayfurther be connected to one or more processors (not shown).

FIG. 18 illustrates a three-dimensional Spin Hall Effect (SHE) MRAMdevice 1800 in accordance with some implementations. The SHE MRAM device1800 is similar to the STT MRAM device 500 explained above, except thatthe SHE MRAM device 1800 includes three terminals 1802, 1804, and 1806,whereas the STT MRAM device 500 includes two terminals (e.g., the bitline 508 and the source line 510, FIG. 5). Like the STT MRAM device 500,the SHE device 1800 includes a plurality of layers (e.g., the referencelayer 102, the spacer layer 104, and the storage layer 106, FIG. 1A)wrapped around a central core 507, thereby forming a three-dimensionalcylindrical (or conical) MTJ structure 501. The MTJ structure 501 isdiscussed in further detail above with reference to FIG. 5 and will notbe repeated here.

The SHE MRAM device 1800 includes a first terminal 1802 and a secondterminal 1804 connected to opposing ends of the core 507, respectively.The first and second terminals are used to create the SHE. A “Spin HallEffect” is a spin accumulation on lateral surfaces of an electriccurrent-carrying sample, where signs of the spin directions are oppositeon opposing boundaries of the electric current-carrying sample. However,a cylindrical electric current-carrying sample (e.g., core 507) does nothave opposing boundaries. Because of this, the current-induced surfacespins wind around a perimeter of the cylindrical electriccurrent-carrying sample. Moreover, when the current direction isreversed, the directions of spin orientation is also reversed (e.g.,switches from a clockwise chirality to a counterclockwise chirality, orvice versa). Accordingly, when the current passes from the firstterminal 1802 to the second terminal 1804, the SHE winds around theperimeter of the core 507 in a first direction (e.g., a firstchirality), and when the current passes from the second terminal 1804 tothe first terminal 1802, the SHE winds around the perimeter of the core507 in a second direction (e.g., a second chirality).

The third terminal 1806 receives (or provides) a STT current, which isthe current discussed above with reference to FIGS. 5-12. For example,in a first flow direction (e.g., inside out), the STT current (e.g.,electron flow 615, FIGS. 6A and 6C) flows radially from the core 507through the plurality of layers. In doing so, the STT current imparts atorque on, at least, a magnetization of an inner layer of the pluralityof layer (e.g., the storage layer 502, FIG. 6C). In another example, ina second flow direction (e.g., outside in), the STT flows from the outerlayer 506 through the plurality of layers towards the core 507. Asdiscussed above, the flow of the STT current through the MTJ structure501 imparts a torque on magnetizations of the two ferromagnetic layers.In some implementations, the third terminal 1806 is also used to readoutthe resistance state or stored memory state of the SHE MRAM device.

Additionally, the first terminal 1802 (or the second terminal 1804,depending on the circumstances, such as the magnetic ground state of thestorage layer 502) provides the SHE current to the core 507, and the SHEcurrent imparts the SHE around a perimeter of the core 507. In doing so,the SHE imparted around the perimeter of the core 507 contributes to thetorque imparted on the magnetization of the first ferromagnetic layer502 by the STT current.

In some implementations, the STT current has a first magnitude and theSHE current has a second magnitude that is different from (e.g., greaterthan) the first magnitude. In addition, in some implementations, amagnitude of the SHE current changes depending on the magnetic groundstate of the SHE MRAM device 1800. For example, when the SHE MRAM device1800 is in the perpendicular magnetic ground state, the SHE current isincreased relative to the SHE current when the SHE MRAM device 1800 isin the vortex magnetic ground state (or vice versa). Moreover, in someimplementations, a pulse duration of the SHE current changes dependingon the magnetic ground state of the SHE MRAM device 1800. For example,when the SHE MRAM device 1800 is in the perpendicular magnetic groundstate, a pulse duration of the SHE current is decreased relative to apulse duration of the SHE current when the SHE MRAM device 1800 is inthe vortex magnetic ground state (or vice versa).

For ease of discussion with regards to FIGS. 19A-19C, the firstcylindrical ferromagnetic layer 502 is the storage layer 502 (alsoreferred to as an inner layer) and the second cylindrical ferromagneticlayer 506 is the reference layer 506 (also referred to as an outer oroutermost layer). However, as noted above, a configuration of thestorage layer 502 and the reference layer 506 may be reversed dependingon the circumstances.

FIGS. 19A-19C illustrate cross-sectional views (taken along line B, FIG.18) of magnetization orientations of the cylindrical MTJ structure 501in accordance with some implementations.

FIG. 19A illustrates a cross-sectional view of the cylindrical MTJstructure 501 having a perpendicular magnetization orientation (e.g.,perpendicular magnetization orientation 710, FIG. 7). Further, FIG. 19Aillustrates the magnetization directions 1902-A, 1902-B of the storageand reference layers in a parallel configuration (although not shown,the magnetization directions 1902-A, 1902-B of the storage and referencelayers could also be in an anti-parallel configuration). FIG. 19Aillustrates a SHE current 1904 passing through the core 507 from thefirst terminal 1802 to the second terminal 1804 (e.g., from top tobottom). The STT current is also being applied, which results in theelectron flow 615 (shown going left to right, but could also go right toleft, depending on the circumstances). The SHE current 1904 creates aSHE 1906 that winds around the perimeter of the core 507. In thisexample, the SHE 1906 (e.g., SHE-electrons) is shown going into thepage, and therefore the SHE 1906 is rotating around the perimeter of thecore 507 in a counterclockwise direction. Accordingly, the rotation ofthe SHE-electrons 1906 is orthogonal to the magnetization 1902 of thestorage layer 502 and the reference layer 506. In such a configuration,the SHE-electrons 1906 provide a spike of orthogonal spin-polarizedelectrons to the storage layer 502 that jumpstart the storage layer's502 precession (e.g., transition) from a first direction ofmagnetization (e.g., upwards) to a second direction of magnetization(e.g., downwards). In some implementations, to maximize the effect ofthe jumpstart, the SHE current 1904 is a short pulse relative to theprecession period of the storage layer 502. For example, the pulse spansfrom approximately 0.1 to 1 nanoseconds. It is noted that the STTcurrent continues to be applied after the SHE current pulse ceases, asshown in FIG. 20. The SHE current 1904 and the STT current with respectthe perpendicular magnetization orientation is discussed in furtherdetail below with reference to FIGS. 20 and 21.

FIG. 19B illustrates a cross-sectional view of the cylindrical MTJstructure 501 having a vortex magnetization orientation (e.g., vortexmagnetization orientation 700, FIG. 7). Further, FIG. 19B illustratesthe magnetization directions 1902-A, 1902-B of the storage and referencelayers in an anti-parallel configuration. For example, the magnetization1902-A of the reference layer 506 is in a first direction (e.g., acounterclockwise chirality) and the magnetization 1902-B of the storagelayer 502 is in a second direction opposite the first directions (e.g.,a clockwise chirality). FIG. 19B illustrates a SHE current 1904 passingthrough the core 507 from the first terminal 1802 to the second terminal1804. In this example, the majority spin polarization of the SHE current1906 or SHE spin polarization is shown coming going into the page, andtherefore the magnetic moment of the SHE-electrons 1906 are rotatingaround the perimeter of the core 507 in a counterclockwise direction.Accordingly, the rotation of the magnetic moment resulting from theSHE-electrons 1906 is (i) aligned with and parallel to the magnetization1902-A of the reference layer 506, and (ii) opposite and parallel to themagnetization 1902-B of the storage layer 502.

In the vortex magnetic ground state, the SHE spin polarization 1906stabilize a ferromagnetic layer when the SHE spin polarization 1906 isaligned with and parallel to the magnetization of the layer (e.g., ifboth have the same chirality), or the SHE spin polarization 1906 tend toswitch a magnetization direction of a ferromagnetic layer when the spinof the SHE-electrons 1906 are opposite and parallel to the magnetizationof the layer (e.g., if both have opposite chiralities). It is notedthat, in some implementations, the SHE current is not a short pulse whenthe ferromagnetic layer is in the vortex magnetization orientation.

FIG. 19C illustrates a cross-sectional view of the cylindrical MTJstructure 501 having the vortex magnetization orientation. FIG. 19Cillustrates a SHE current 1904 passing through the core 507 from thesecond terminal 1804 to the first terminal 1802. In this example, theSHE spin polarization 1906 is shown coming out of the page, andtherefore the spin of the SHE electrons 1906 is rotating around theperimeter of the core 507 in a clockwise direction. Accordingly, therotation of the SHE-spin polarization 1906 is (i) opposite and parallelto the magnetization 1902-A of the reference layer 506, and (ii) alignedwith and parallel to the magnetization 1902-B of the storage layer 502.It is noted that the results shown in FIGS. 19B-19C are merely two ofthe resulting spin hall effects. One skilled in the art will appreciatethat in some instances the resulting spin hall effects shown in FIGS.19B-19C may be reversed.

For convenience, FIGS. 20 and 21 have been reproduced from A. Van denBrink et al. “Spin-Hall-assisted magnetic random access memory,” Appl.Phys. Lett. 104, 012403 (2014).

FIG. 20 illustrates representations of a ferromagnetic layer (e.g.,storage layer 106, FIG. 1A) switching from a first magnetizationdirection to a second magnetization direction in accordance with someimplementations (“1” to “−1”) (e.g., switching from a firstmagnetization direction (e.g., upwards) to a second magnetizationdirection (e.g., downwards)). FIG. 20 includes three differentmagnetization representations 2002, 2004, and 2006 of a ferromagneticlayer in a perpendicular magnetic ground state. Polarizationrepresentation 2002 shows a polarization (e.g., magnetization) of theferromagnetic layer switching from “1” to “−1” using only STT current(shown as J_(STT)). In diagram 2010, the polarization of theferromagnetic layer using J_(STT) alone remains initially near “1.”After approximately 4-5 nanoseconds of applying the STT current to theferromagnetic layer, the polarization gradually switches from “1” to“−1.” In total, the switch from “1” to “−1” spans approximately 8nanoseconds when using J_(STT) alone. The result illustrated inpolarization representation 2002 in some instances corresponds to theSTT MRAM device 500 (FIG. 5) and the transition from FIG. 6A to FIG. 6B(e.g., a transition from a parallel configuration to an anti-parallelconfiguration).

Polarization representation 2004 shows a polarization of theferromagnetic layer attempting to switch from “1” to “−1” using only SHEcurrent (shown an J_(SHE)). In diagram 2010, the polarization of theferromagnetic layer remains near “1,” and is unable to switch from “1”to “−1.” The SHE current alone is generally unable to switch thepolarization of the ferromagnetic layer from “1” to “−1” because, in theperpendicular magnetic ground state, the SHE current creates a SHEaround the perimeter of the core 507 that is orthogonal to amagnetization of the ferromagnetic layer, as illustrated and describedabove with reference to FIG. 19A. Thus, after an initial brief period oftime in which the SHE facilitates switching, the SHE begins to impedethe switching process or simply does nothing.

Polarization representation 2006 shows a polarization of theferromagnetic layer switching from “1” to “−1” using STT current and theSHE current simultaneously, at least initially. In diagram 2010, the SHEcurrent and the STT current are initially applied to the ferromagneticlayer simultaneously. As discussed above with reference to FIG. 19A, theSHE current is applied as a pulse, whereas the STT current iscontinuously applied. The SHE current pulse jumpstarts the switchingprocesses, as shown by the polarization of the ferromagnetic layeralmost immediately moving downwards away from “1.” Even after the SHEcurrent pulse is stopped, the polarization of the ferromagneticcontinues downwards, and the switch from “1” to “−1” spans approximately2 nanoseconds. Accordingly, the switching time for the polarizationrepresentation 2006 is substantially less than the switching time forthe polarization representation 2002. Moreover, in some implementations(not shown), the current density of the STT current in the polarizationrepresentation 2006 is less than the current density of the STT currentin the polarization representation 2002. Thus, in some implementations,using the STT current and the SHE current simultaneously, at leastinitially, to switch the polarization of the ferromagnetic layer (i)reduces a current density of the STT current, and (ii) facilitatesfaster switching of the ferromagnetic layer from a first polarization toa second polarization.

In some implementations, a current density of the SHE current is changeddepending on a pulse length of the applied SHE current. FIG. 21 providesa diagram showing a relationship between SHE current density and SHEpulse duration.

FIG. 22 is a schematic diagram of relative resistances for the SHE-MRAMdevice of FIG. 18 in accordance with some implementations. Terminal Ccorresponds to the first terminal 1802, terminal A corresponds to thesecond terminal 1804, and terminal B corresponds to the third terminal1806. As shown, each terminal is at a voltage (e.g., terminal C is atvoltage V_(C), terminal A is at voltage V_(A), and terminal B is atvoltage V_(B)). In some implementations, one or more of the voltagesdiffer from each other.

The schematic diagram further provides relative resistances fordifferent portions of the MRAM device (e.g., a resistance through theMTJ structure (R_(MTJ)), and two resistances through the core(R_(CORE,A) and R_(CORE,C)). As shown, the R_(MTJ) is far greater thanboth R_(CORE,A) and R_(CORE,C). This occurs because voltage passingthrough the MTJ has to pass through the plurality of layers, includingthe spacer layer which is an insulator. In contrast, the core is aconductive metal, which provides little resistance to a current passingthrough it. Consequently, the current/voltage passed through the R_(MTJ)is small compared to the currents passed through R_(CORE,A) andR_(CORE,C). Thus, from Kirkhoff's law, I_(CORE,A)≈I_(CORE,C,) andtherefore, I_(CORE,A) and I_(CORE,C) can be treated as if they are thesame current: the “Spin Hall current” (e.g., I_(SHE)>>I_(STT)). In thisexample, one assumes a current source at terminal A and terminals B andC are grounded. Accordingly, I_(SHE) and I_(STT) are not independentlyset up. Further, in this example, to first approximation, I_(C)>>I_(B),R_(MTJ)>>R_(COREA) and R_(MTJ)>>RcoreB. I_(STT)˜I_(B) and I_(SHE)˜I_(C)because I_(B) is much smaller.

In some implementations, terminal A is grounded and two current sources,one each at terminals B (I_(B)) and C (I_(C)), are used (e.g., eachpower supply supplies a fixed current with the voltage at terminal B andC floating). Accordingly, the current going through R_(CORE,A) would bethe sum of the currents I_(B) and I_(C). In this example, I_(STT)=I_(B).In some implementations, there is no well-defined ABC node and I_(SHE)is a continuous function of the position along the core. Accordingly,determining I_(SHE) would be more complicated since I_(SHE)=I_(C) abovethe node connecting ABC and I_(SHE)=I_(C)+I_(B) below the nodeconnecting ABC.

Referring to FIG. 21, as an example, J_(SHE)˜30 MA/cm² and J_(STT)˜1MA/cm². In this example, using a Ta core with resistivity 1.3E-5 Ωcm, awidth of 10 nm and a height of ˜30 nm, the resistance of the Ta corewould be ˜12 Ω. Further, in this example, using a MgO barrier with an RAproduct of 10 Ω.μm², the resistance of the barrier would be ˜10 kΩ.According to this example, in order to obtain an I_(SHE) of ˜7.4E-5 A(J_(SHE) multiplied by the cross section of the core) voltages of ˜1 mVto terminal C and ˜100 mV to terminal B are applied. The timing of thosepulses could be less than ˜10 ns. An example pulse would be 1 mV SHEpulse for ˜2 ns and a simultaneous 10 ns STT 100 mV pulse.

In light of these principles, we now turn to certain implementations.

In accordance with some implementations, a magnetic memory device isprovided (e.g., STT-MRAM device 500, FIG. 5). The magnetic memory deviceincludes a cylindrical core (e.g., core 507, FIG. 5), a firstcylindrical ferromagnetic layer (e.g., ferromagnetic layer 502, FIG. 5)that surrounds the cylindrical core, a spacer layer (e.g., spacer layer504, FIG. 5) that surrounds the first cylindrical ferromagnetic layer,and a second cylindrical ferromagnetic layer (e.g., ferromagnetic layer506, FIG. 5) that surrounds the spacer layer. The cylindrical core, thefirst cylindrical ferromagnetic layer, the spacer layer, and the secondcylindrical ferromagnetic layer collectively form a magnetic tunneljunction (e.g., magnetic tunnel junction structure 501, FIG. 5).

In some implementations, the cylindrical core, the first cylindricalferromagnetic layer, the spacer layer, and the second cylindricalferromagnetic layer are coaxial with one another (e.g., as shown in FIG.5). Additionally, in some implementations, heights of the cylindricalcore, the first cylindrical ferromagnetic layer, the spacer layer, andthe second cylindrical ferromagnetic layer substantially match oneanother (e.g., as shown in FIG. 5).

In some implementations, the magnetic memory device further includes afirst terminal lead connected to the cylindrical core (e.g., source line510, FIG. 5) and a second terminal lead connected to the secondcylindrical ferromagnetic layer (e.g., bit line 508, FIG. 5), or viceversa.

In some implementations, the first cylindrical ferromagnetic layer has afirst set of characteristics and the second cylindrical ferromagneticlayer has a second set of characteristics that at least partially differfrom the first set of characteristics. In some implementations, amagnetic ground state of the first and second cylindrical ferromagneticlayers is based, at least in part, on characteristics of the first andsecond cylindrical ferromagnetic layers, respectively. Examples ofmagnetic ground states are provided above with reference to FIGS. 7A-7C.

In some implementations, the first and second sets of characteristicsinclude: (i) thicknesses of the first and second cylindricalferromagnetic layers and (ii) heights of the first and secondcylindrical ferromagnetic layers, respectively. In some implementations,the first and second sets of characteristics further include layercomposition (e.g., single layer versus multiple sublayers), exchangeenergy, saturation magnetization, and uniaxial anisotropy. Further, insome implementations, the magnetic ground state of the first and secondcylindrical ferromagnetic layers is further based on characteristics ofthe cylindrical core. For example, the characteristics of thecylindrical core include: (i) a radius of the cylindrical core and (ii)a height of the cylindrical core. FIGS. 8A-8B provide phase diagrams 800and 810 showing the relationship between certain characteristics andvarious magnetic ground states.

In some implementations, the first cylindrical ferromagnetic layer is astorage layer (e.g., storage layer 106, FIG. 1A) and the secondcylindrical ferromagnetic layer is a reference layer (e.g., referencelayer 102, FIG. 1A). Alternatively, in some implementations, the firstcylindrical ferromagnetic layer is the reference layer and the secondcylindrical ferromagnetic layer is the storage layer.

In some implementations, a magnetization direction of the firstcylindrical ferromagnetic layer mirrors a magnetization direction of thesecond cylindrical ferromagnetic layer when the magnetic memory deviceis in a first resistance state (e.g., parallel resistance states shownin FIGS. 6A and 6C). Alternatively, in some implementations, amagnetization direction of the first cylindrical ferromagnetic layer isopposite the magnetization direction of the second cylindricalferromagnetic layer when the magnetic memory device is in a secondresistance state (e.g., anti-parallel configurations shown in FIGS. 6Band 6D).

Additionally, in some implementations, the first and second cylindricalferromagnetic layers are magnetized along an axis (e.g., axis 704) wheneach layer is in a first magnetic ground state (e.g., in theperpendicular magnetic ground state, magnetizations 602, 604, and 606point upwards or downwards in FIGS. 6A-6B). Alternatively, in someimplementations, the first and second cylindrical ferromagnetic layersare magnetized about the axis when each layer is in a second magneticground state different from the first magnetic ground state (e.g., inthe vortex magnetic ground state, magnetizations 612, 614, and 616either have a clockwise chirality or a counterclockwise chirality).

In some implementations, the first and second cylindrical ferromagneticlayers are in a first magnetic state (e.g., the perpendicular magneticground state) when a ratio between respective heights and thicknesses ofthe two layers satisfy a threshold, and the first and second cylindricalferromagnetic layers are in a second magnetic state (e.g., the vortexmagnetic ground state) when the ratio between the respective heights andthicknesses of the two layers do not satisfy the threshold. FIGS. 8A-8Bprovide phase diagrams 800 and 810 showing the relationship betweenheights and thicknesses, and various magnetic ground states.

In some implementations, the cylindrical core is a non-magnetic metaland the cylindrical core is configured to receive a current. Forexample, the core 507 may receive a current from the source line 510(FIG. 5). Further, in some implementations, received current flowsradially through the first cylindrical ferromagnetic layer and thespacer layer towards the second cylindrical ferromagnetic layer. Forexample, the current (e.g., electron flow 615) starts in the core 507and moves radially to the second ferromagnetic layer 506 (FIG. 6C).Moreover, radial flow of the current imparts a torque at least on amagnetization of the first cylindrical ferromagnetic layer (and in someimplementations imparts a torque on the second cylindrical ferromagneticlayer). In some implementations, the magnetization of the firstcylindrical ferromagnetic layer changes from a first direction to asecond direction when the current satisfies a threshold. For example,FIGS. 6A-6B illustrate the magnetization of the first cylindricalferromagnetic layer 502 changing from a first direction (e.g., upwards)to a second direction (e.g., downwards) when the current satisfies thethreshold (e.g., energy barrier 1016, FIG. 10B). In doing so, the firstand second ferromagnetic layers in FIG. 6B are in an anti-parallelstate. In another example, FIGS. 6C-6D illustrate the magnetization ofthe first cylindrical ferromagnetic layer 502 changing from a firstdirection (e.g., counterclockwise chirality) to a second direction(e.g., clockwise chirality) when the current satisfies the threshold(e.g., energy barrier 1006, FIG. 10A). In doing so, the first and secondferromagnetic layers in FIG. 6D are in an anti-parallel state. It isnoted that in some implementations the threshold corresponding to FIGS.6A-6B differs from the threshold corresponding to FIGS. 6C-6D.

The STT-MRAM device 500 is discussed in further detail above withreference to FIGS. 5-12.

In accordance with some implementations, another magnetic memory deviceis provided (e.g., SHE-MRAM device 1800, FIG. 18). The magnetic memorydevice includes a core (e.g., core 507, FIG. 18) and a plurality oflayers that surround the core in succession. For example, with referenceto FIG. 18, the plurality of layers includes a first ferromagnetic layer502, followed by a spacer layer 504, followed by a second ferromagneticlayer 506. The first ferromagnetic layer 502 is sometimes referred tobelow as the “inner layer,” and the second ferromagnetic layer 506 issometimes referred to below as the “outer layer.” In someimplementations, the inner layer is the storage layer 106. Additionally,the core, the first ferromagnetic layer, the spacer layer, and thesecond ferromagnetic layer collectively form a magnetic tunnel junction(e.g., magnetic tunnel junction structure 501, FIG. 18).

The magnetic memory device further includes a first input terminalcoupled to the core. The first input terminal is configured to receive afirst current (also referred to herein as STT current and J_(STT)). Insome implementations, the first current flows radially from the corethrough the plurality of layers and the radial flow of the first currentimparts a torque on, at least, a magnetization of an inner layer of theplurality of layers (e.g., electron flow 615, FIG. 6C). The STT currentis described in further detail above with reference to FIGS. 1-12.

The magnetic memory device further includes a second input terminalcoupled to the core. The second input terminal is configured to receivea second current (also referred to herein as SHE current and J_(SHE)).The second current imparts a Spin Hall Effect (SHE) around a perimeterof the core, and the SHE imparted around the perimeter of the corecontributes to the torque imparted on the magnetization of the innerlayer by the first current. For example, with reference to FIG. 19A, theSHE current 1904 flows from the first terminal 1802 to the secondterminal 1804 (or vice versa), and in doing so, imparts the SHE 1906around the perimeter of the core 507. Further, the SHE 1906 is adjacentto the inner layer 502, and as a result, SHE-electrons of the SHE 1906contribute to the torque imparted on the magnetization of the innerlayer by the first current.

In some implementations, the first and second input terminals are thesame terminal. For example, with reference to FIG. 18, the first andsecond input terminals may be examples of the first terminal 1802 (orthe second terminal 1804). Alternatively, in some implementations, thefirst and second inputs terminals are not the same terminal. Forexample, with reference to FIG. 18, the first input terminal may be anexample of the first terminal 1802 and the second input terminal may bean example of the second terminal 1804 (or vice versa). In anotherexample, the first input terminal may be an example of the thirdterminal 1806 and the second input terminal may be an example of thefirst terminal 1802 or the second terminal 1804.

In some implementations, when the magnetic memory device is in a firstmagnetic ground state (e.g., a perpendicular magnetic ground state 710,FIG. 7B), the inner layer is magnetized in a first direction and the SHEimparted around the perimeter of the core flows in a second directionthat is substantially orthogonal to the first direction. For example,with reference to FIG. 19A, the magnetization 1902-B of the inner layer502 is upwards and the SHE spin polarization 1906 flows around theperimeter of the core 507 with a clockwise chirality, which issubstantially orthogonal to the upwards direction of the magnetization1902-B.

Further, in some implementations, when the magnetic memory device is inthe first magnetic ground state: (i) the first input terminal receivesthe first current for a first period of time and (ii) the second inputterminal receives the second current for a second period of time. Insome implementations, the second period of time is less than the firstperiod of time. For example, with reference to FIG. 20, the J_(SHE) (thesecond current) is applied for approximately 0.5 nanoseconds and theJ_(STT) (the first current) is applied for some period of time greaterthan 0.5 nanoseconds. In some implementations, the second current isapplied as a pulse. FIG. 21 provides additional examples of the secondperiod of time.

In some implementations, the first input terminal is further configuredto receive the first current at a first time and the second inputterminal is also configured to receive the second current at the firsttime. For example, with reference again to FIG. 20, the J_(SHE) (thesecond current) and the J_(STT) (the first current) are, at leastinitially, applied simultaneously.

In some implementations, when the magnetic memory device is in the firstmagnetic ground state, the magnetization of the inner layer switchesfrom the first direction to a third direction after a third period oftime when the SHE is not imparted around the perimeter of the core atall. For example, with reference again to FIG. 20, when only the J_(STT)(the first current) is applied, the magnetization of the inner layerswitches from the first direction to the third direction afterapproximately 8 nanoseconds. In some embodiments, the first, second, andthird directions are all distinct directions.

In contrast, when the magnetic memory device is in the first magneticground state, the magnetization of the inner layer switches from thefirst direction to the third direction after a fourth period of timewhen the SHE is imparted around the perimeter of the core for the secondperiod of time. For example, with reference again to FIG. 20, when boththe J_(STT) and the J_(SHE) are applied, the magnetization of the innerlayer switches from the first direction to the third direction afterapproximately 2 nanoseconds. Thus, the fourth period of time is lessthan the third period of time.

In some implementations, when the magnetic memory device is in a secondmagnetic ground state (e.g., the vortex magnetic ground state 700, FIG.7A): (i) the inner layer is magnetized in a first chirality, (ii) theSHE imparted around the perimeter of the core has a second chiralitythat is opposite to the first chirality; and (iii) the SHE is combinedwith the torque imparted on the magnetization of inner layer by thefirst current. For example, with reference to FIG. 19B, a magnetization1902-B of the inner layer 502 is coming out of the page (e.g., aclockwise chirality) and the SHE spin polarization 1906 is going intothe page (e.g., a counterclockwise chirality). In such a configuration,the SHE spin polarization 1906 is combined with the torque imparted onthe magnetization of the inner layer by the first current. FIG. 19Cprovides an example of the SHE polarization 1906 coming out of the page,which is caused by switching a direction of the current 1904 (asdiscussed above). In FIG. 19C, the magnetization of the inner layer 502is also coming out of the page (e.g., the inner layer is magnetized in afirst chirality and the SHE torque imparted around the perimeter of thecore also has the first chirality). In such a configuration, the SHEstabilizes the magnetization of inner layer.

In some implementations, when the magnetic memory device is in thesecond magnetic ground state, the magnetization of the inner layerswitches from the first chirality to a second chirality when thecombined torque imparted on the magnetization of the inner layersatisfies a threshold (e.g., energy barrier 1006, FIG. 10A).

In some implementations, the first current has a first magnitude and thesecond current has a second magnitude. In some implementations, thesecond magnitude is greater than the first magnitude (e.g., J_(SHE) isgreater than J_(STT), FIG. 20). Alternatively, in some implementations,the second magnitude is substantially equal to the first magnitude.

In some implementations, the magnetic memory device further includes anoutput terminal coupled to an outer layer of the plurality of layers,where the output terminal is configured to provide a current readout toa readout component of the magnetic memory device.

Although some of various drawings illustrate a number of logical stagesin a particular order, stages that are not order dependent may bereordered and other stages may be combined or broken out. While somereordering or other groupings are specifically mentioned, others will beobvious to those of ordinary skill in the art, so the ordering andgroupings presented herein are not an exhaustive list of alternatives.Moreover, it should be recognized that the stages could be implementedin hardware, firmware, software or any combination thereof

It will also be understood that, although the terms first, second, etc.are, in some instances, used herein to describe various elements, theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. For example, a first electronicdevice could be termed a second electronic device, and, similarly, asecond electronic device could be termed a first electronic device,without departing from the scope of the various describedimplementations. The first electronic device and the second electronicdevice are both electronic devices, but they are not the same type ofelectronic device.

The terminology used in the description of the various describedimplementations herein is for the purpose of describing particularimplementations only and is not intended to be limiting. As used in thedescription of the various described implementations and the appendedclaims, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “includes,” “including,” “comprises,” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

As used herein, the term “if” is, optionally, construed to mean “when”or “upon” or “in response to determining” or “in response to detecting”or “in accordance with a determination that,” depending on the context.Similarly, the phrase “if it is determined” or “if [a stated conditionor event] is detected” is, optionally, construed to mean “upondetermining” or “in response to determining” or “upon detecting [thestated condition or event]” or “in response to detecting [the statedcondition or event]” or “in accordance with a determination that [astated condition or event] is detected,” depending on the context.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific implementations. However, theillustrative discussions above are not intended to be exhaustive or tolimit the scope of the claims to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The implementations were chosen in order to best explain theprinciples underlying the claims and their practical applications, tothereby enable others skilled in the art to best use the implementationswith various modifications as are suited to the particular usescontemplated.

1. A magnetic memory device comprising: a core; a plurality of layersthat surround the core in succession; a first input terminal coupled tothe core, the first input terminal being configured to receive a firstcurrent, wherein: the first current flows radially from the core throughthe plurality of layers; and the radial flow of the first currentimparts a torque on, at least, a magnetization of an inner layer of theplurality of layers; and a second input terminal coupled to the core,the second input terminal being configured to receive a second current,wherein: the second current imparts a Spin Hall Effect (SHE) around aperimeter of the core; and the SHE imparted around the perimeter of thecore contributes to the torque imparted on the magnetization of theinner layer by the first current.
 2. The magnetic memory device of claim1, wherein: when the magnetic memory device is in a first magneticground state: the inner layer is magnetized in a first direction; andthe SHE imparted around the perimeter of the core flows in a seconddirection that is substantially orthogonal to the first direction. 3.The magnetic memory device of claim 2, wherein: when the magnetic memorydevice is in the first magnetic ground state: the first input terminalreceives the first current for a first period of time; and the secondinput terminal receives the second current for a second period of time,the second period of time is less than the first period of time.
 4. Themagnetic memory device of claim 3, wherein: the first input terminal isfurther configured to receive the first current at a first time; and thesecond input terminal is further configured to receive the secondcurrent at the first time.
 5. The magnetic memory device of claim 3,wherein: when the magnetic memory device is in the first magnetic groundstate: the magnetization of the inner layer switches from the firstdirection to a third direction after a third period of time when the SHEis not imparted around the perimeter of the core at all; themagnetization of the inner layer switches from the first direction tothe third direction after a fourth period of time when the SHE isimparted around the perimeter of the core for the second period of time;and the fourth period of time is less than the third period of time. 6.The magnetic memory device of claim 2, wherein: when the magnetic memorydevice is in a second magnetic ground state: the inner layer ismagnetized in a first chirality; the SHE imparted around the perimeterof the core has a second chirality that is opposite to the firstchirality; and the SHE is combined with the torque imparted on themagnetization of the inner layer by the first current. (CurrentlyAmended) The magnetic memory device of claim 6, wherein: when themagnetic memory device is in the second magnetic ground state, themagnetization of the inner layer switches from the first chirality to asecond chirality when the combined torque imparted on the magnetizationof the inner layer satisfies a threshold.
 8. The magnetic memory deviceof claim 2, wherein: when the magnetic memory device is in a secondmagnetic ground state: the inner layer is magnetized in a firstchirality; the SHE imparted around the perimeter of the core has thefirst chirality; and the SHE stabilizes the magnetization of innerlayer.
 9. The magnetic memory device of claim 1, wherein: the firstcurrent has a first magnitude; the second current has a secondmagnitude; and the second magnitude is greater than the first magnitude.10. The magnetic memory device of claim 1, wherein: at least two of theplurality of layers are composed of ferromagnetic material; and adifferent one of the plurality of layers is composed of a dielectricmaterial.
 11. The magnetic memory device of claim 1, further comprisingan output terminal coupled to an outer layer of the plurality of layers,the output terminal being configured to provide a current readout to areadout component of the magnetic memory device.
 12. The magnetic memorydevice of claim 1, wherein: the plurality of layers includes a firstferromagnetic layer, a spacer layer, and a second ferromagnetic layer;and the inner layer is the first ferromagnetic layer.
 13. The magneticmemory device of claim 12, wherein: the first ferromagnetic layersurrounds the core; the spacer layer surrounds the first ferromagneticlayer; and the second ferromagnetic layer surrounds the spacer layer,wherein the core, the first ferromagnetic layer, the spacer layer, andthe second ferromagnetic layer collectively form a magnetic tunneljunction.
 14. The magnetic memory device of claim 13, wherein the coreis conical in shape.
 15. The magnetic memory device of claim 13, whereinthe core is cylindrical in shape.
 16. The magnetic memory device ofclaim 1, wherein the core is a metal core.
 17. The magnetic memorydevice of claim 1, wherein: the core comprises first and second opposingends; the first input terminal is coupled to the first end of the core;the magnetic memory device further comprises an output terminal coupledto the second end of the core; and the second current travels from thefirst input terminal, through the core, to the output terminal, therebyforming the SHE.